Matthias Bastian

Dolores Morales

Rinaldo Poli
email: poli@u-bourgogne.fr

Philippe Richard

Helmut Sitzmann

Prof Rinaldo Poli

Synthesis of new half sandwich tetrachloro derivatives of molybdenum(

Keywords: Molybdenum, tungsten, bulky cyclopentadienyl ligands, organometallic synthesis, X-ray structure

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Half-sandwich halide compounds are useful intermediates for the preparation of a large variety of organometallic products [1]. Cyclopentadienyl ligands carrying large substituents have been used to study the influence of steric bulk on metallocenes of the transition metals [2][3][4][5][6][7][8][9][10]. They are capable of stabilizing unusual unsaturated configurations and of tuning the reactivity of other co-ligands via their control of the steric environment around the metal [11][12][13][14][15]. It therefore desirable to develop new half-sandwich halide derivatives containing sterically encumbering cyclopentadienyl rings. The pentaethylcyclopentadienyl ( V Cp) ligand [16,17] has recently been utilized for the preparation of ferrocene and ferrocenium analogues [8] and the half-sandwich Mo(V) complex ( V Cp)MoCl4 [18]. In the latter report, it was shown that while the electronic effect of the V Cp ligand is equivalent to that of the pentamethyl analogue Cp*, the greater steric encumberance exerts a kinetic control of the protonation reaction and on the dihydrogen elimination reaction from the protonated products. We are therefore interested in the development of a larger variety of half sandwich Mo(V) species, and also their W(V) counterparts, for the further development of this sterically protected hydride chemistry.

In this contribution, we report the synthesis of the tetrachloro Mo(V) complex containing the tetraisopropylcyclopentadienyl ( 4 Cp) ligand and of both the V Cp and 4 Cp analogues of tetrachloro W(V), including the X-ray structure of V CpWCl4.

Experimental

All manipulations were carried out under an inert atmosphere of argon. Solvents were dried by standard methods (THF, Et2O, toluene and pentane over Na/ benzophenone, CH2Cl2 over P2O5, CH3CN over molecular sieve), followed by distillation under argon. The instrumentation used for the spectroscopic characterization was as follows: 1 H NMR, Bruker AC 200 or AMX 400; IR, Perkin-Elmer IFS 66 and Perkin Elmer 16 PC; EPR, Bruker ESP 300 (X band). Elemental analyses were performed by the LSEO analytical services with a Fisons EA 1108 apparatus and by the Fachbereich Chemie Kaiserslautern analytical service with a Perkin-Elmer 240 or 400 CHN analyzer. Compounds PhICl2 [19], Li V Cp [16], and Na 4 Cp [6] were prepared by literature methods.

Synthesis of 4 CpMo(CO)3H, 1. Hexacarbonylmolybdenum (4.51 g, 17.1 mmol) and sodium tetraisopropylcyclopentadienide (4.817 g, 18.7 mmol) were suspended together in 100 mL of THF. The mixture was refluxed for 16 h, during which time it transformed into an orange solution. The IR spectrum of this solution showed bands at 1889 s, 1785 s and 1734 s cm -1 and the complete consumption of the starting material. This solution was subsequently added slowly to an aqueous solution of Fe3(SO4)2 (6.8 g, 17 mmol, in 2 L of degased water).

After stirring for 1 h, the resulting orange precipitate was filtered in air through a Buchner funnel. The solid was purified by dissolution in pentane and filtration through alumina, leaving a brown insoluble deposit on the column. Subsequent removal of the solvent by evaporation under reduced pressure and prolonged drying left a spectroscopically pure oil. Yield: 4.09 g (58%). The spectroscopic (IR, 1 H NMR) properties match those previously reported in the literature [20].

Synthesis of 4 CpMoCl4, 2.

To a solution of V CpMo(CO)3H (4.09 g, 9.86 mmol) in 40 mL of dichloromethane was added in small portions a solution of PhICl2 (6.78 g, 24.7 mmol) in 50 mL of CH2Cl2 by cannula. During the addition, the color of the solution changed to purple, while vigorous gas evolution ensued and a purple solid precipitated. The reaction mixture was further stirred for 1 h at room temperature. After filtration, the solid was washed with pentane (5 x 20 mL) and dried under vacuum. Yield 1.97 g (42.4%). EPR (CH2Cl2): singlet with Mo satellites, g = 1.989, aMo = 38.9. Anal. Calcd. for C17H29Cl4Mo: C, 43.34; H, 6.20. Found: C, 43.81; H, 6.25.

Synthesis of V CpW(CO)3(CH3), 3.

To a suspension of hexacarbonyltungsten (2.287 g, 6.5 mmol) in 50 mL of THF was added by cannula a solution of lithium pentaethylcyclopentadienide in 25 ml of THF at room temperature. The yellow-brown reaction mixture was refluxed to yield a red-orange solution after ca. 12h. After cooling down to room temperature, methyl iodide (0.40 mL, 0.92 g, 6.5 mmol) was added with a syringe. During the addition, the color immediately changed to red-brown. The reaction mixture was refluxed for an additional hour. The solvent was then removed to obtain a brown precipitate which was extracted into CH2Cl2 and filtered through Celite. Evaporation of the filtrate to dryness yielded 3.016 g (95 %) of an orange-yellow solid. IR (pentane, cm

Synthesis of V CpWCl4, 4.

To a solution of compound 3 (2.09 g, 4.28 mmol) in 20 mL of dichloromethane was added in small portions a solution of PhICl2 (4.12 g, 15.0 mmol) in 40 mL of CH2Cl2 by cannula. During the addition, the color of the solution changed from brown to dark green, while vigorous gas evolution ensued. The reaction mixture was refluxed for 5 h yielding an orange-brown solution. The solution was then treated with 25 ml of THF to destroy the excess of PhICl2. The solvents were then removed by evaporation under reduced pressure and the olive precipate was extracted into dichloromethane and filtered over Celite. Evaporation of the filtrate to dryness left a brown solid (1.17 g, 51.5 %). Single crystals of this compound were obtained from a dichloromethane solution at -80° C in 12 h. Anal. Calcd. for C15H25Cl4W: C, 33.93; H, 4.75. Found: C, 33.57; H, 4.57. EPR (CH2Cl2): broad singlet, g = 1.958, width (peak-to-peak) = 13 G, aW = 64 G.

Synthesis of 4 CpW(CO)3(CH3), 5.

To a suspension of hexacarbonyltungsten (1.76 g, 5.0 mmol) in 30 ml of THF was added dropwise a solution of sodium tetraisopropylcyclopentadienide (1.282 g, 5 mmol) in 10 ml of THF at room temperature. The yellow-brown reaction mixture was refluxed to yield a red-brown solution after ca. 14 h. After cooling down to room temperature, methyl iodide (0.31 mL, 0.71 g, 5.0 mmol) was added with a syringe. The reaction mixture was refluxed for an additional hour. The solvent was then removed to obtain a brown precipitate which was extracted into CH2Cl2 and filtered. General. Both intensity data were collected on a Nonius Kappa CCD diffractometer at 110 K. The structures were solved by a Patterson search program and refined by full-matrix least-square methods (SHELX-97) [START_REF] Sheldrick | SHELX97 (Includes SHELXS97 and SHELXL97), Release 97-2, Programs for Crystal Structure Analysis[END_REF] with the aid of the WINGX program [START_REF] Farrugia | [END_REF]. The crystal data and refinement parameters for both compounds are collected in Table 1.

<Table 1>

Compound V CpWCl4, 4. A single crystal was selected from the -80° crystallization solution after removing the mother liquor by cannula transfer. The crystals were washed with cold pentane and dried by purging with argon. Prior to mounting on the diffractometer, the crystal was stored at -80° C to avoid decomposition. The refinement was rather unstable and led to non-positive defined adp for some atoms. A careful inspection of the data set indicated that the hkl layers with k odd exhibited a much lower mean intensity than the even ones. For example: (k,<F 2 >) = (0, 189), (1,6), (2,256), (3,5), (4, 253), (5,6) etc. Skipping these odd layers led to the new space group Pbcm with cell parameters a = 7.8925(5) Å, b = 16.0106 (7) Å, c = 14.2596(9) Å. In this space group, the molecule is located on a mirror plane, which could explain the pseudo-symmetry problem encountered. Nevertheless, a refinement in this group led to non-realistic adp and strong residual peaks in the last Fourier difference map. We present the results in the Pbca space group with an isotropic refinement of the carbon atoms.

Hydrogen atoms were included in a riding model. Selected bond distances and angles are listed in Table 2.

<Table 2>

Compound 4 CpW(CO)3(CH3), 5. A single crystal could be obtained from a benzene solution at room temperature. The mother liquor was decanted off and the crystals were dried in a nitrogen purge. Non-hydrogen atoms were anisotropically refined and hydrogen atoms, found in the last Fourier difference maps, were refined freely. Selected bond distances and angles are listed in Table 2.

Results and Discussion

The synthetic scheme for the tetrachloro compounds reported in this paper parallel those reported previosuly by Schrock [23], Green [24] and ourselves [START_REF] Keogh | Synthetic Methods of Organometallic and Inorganic Chemistry[END_REF][START_REF] Gordon | Synthetic Methods of Organometallic and Inorganic Chemistry[END_REF] for the Cp* and other cyclopentadienyl derivatives of Mo and W, including V CpMoCl4 [18]. These are illustrated in Scheme 1, where (Ring) is a generic cyclopentadienyl ligand and M' is an alkali metal, usually Li, Na or K. Salts of the appropriate cyclopentadienyl derivative react with the hexacarbonyl starting material to afford the corresponding cyclopentadienyltricarbonyl anion.

In the Schrock method, the latter anion is quenched by methyl iodide to afford a tricarbonyl methyl product which is subsequently transformed to the tetrachloro product by use of 2.5 equivalents of PCl5. In the method used by Green and us, on the other hand, the anion is transformed to the neutral dimer by the ferric oxidation which was first illustrated by Manning [START_REF] Birdwhistell | [END_REF]. This neutral species can then be transformed to the tetrachloro product by the use of only 2 equivalents of either PCl5 or the convenient alternative chlorinating agent PhICl2. For reasons of atom economy, we initially adopted the dimer route for the preparation of 4 CpMoCl4. We were surprised, however, to find that the product of the aqueous ferric oxidation was not the expected dimer but rather the corresponding Mo(II) hydride species, 4 CpMo(CO)3H, 1, which was recovered in a relatively good yield. No trace of the dimer was evident from the IR spectrum of the crude product. This reactivity pattern may result from the steric shielding exerted by the bulky cyclopentadienyl, which slows down the rate of thermodynamically favorable processes. This effect is apparently stronger for the outer sphere electron transfer from Fe 3+ (aq) with respect to the metal protonation process. The protonation should be a favorable process also for less encumbered anions, since the pKa of, for instance, CpMo(CO)3H is 13.9 in acetonitrile [28] (corresponding to ca. 6.4 in water) [29] and derivatives with alkyl-substituted cyclopentadienyl ligands are naturally expected to be less acidic. However, the success of the Manning procedure for the synthesis of the Cp and even Cp* dimers shows that the electron transfer process is faster in those cases.

Scheme 1 (actual publication size: single column)

The properties of the hydride compound 1 are as expected, with a typical pattern of CO stretching vibrations in the IR and a highly shielded 1 H NMR resonance for the hydride proton [30,31]. The clean conversion shown by Schrock for the methyl compound to the tetrachloro product made us suspect that the hydride compound could also be a convenient starting material for the same transformation according to the same stoichiometry, with elimination of HCl (see Equation 1). Indeed, this turned out to be the case, compound

1 2
In view of the above complications, the synthesis of the tungsten compounds The W(V) tetrachloro compounds are characterized by an EPR signal whose parameters are independent on the nature of the ring. Their linewidth (ca. 13 G peak-to-peak) is sharper than that observed previously for the Cp* analogue (50 G) [23], allowing the clear observation of the metal satellites. The hyperfine coupling constant to the 183 W nucleus (14.4% abundance) is 62 G.

<Table 3>

The 1 H NMR data on the three 4 Among the alternative arrangements shown in Scheme Hindered rotation of isopropyl groups is not commonly observed in tetraisopropylcyclopentadienyl complexes at room temperature, except for the extremely congested octaisopropylferrocene [6]. Therefore, it seems more likely that the isopropyl groups are freely rotating in solution for compounds 1 and 5. This hypothesis is equally in agreement with the observation of four distinct doublet resonances of equal intensity for the isopropyl methyl protons in these compounds. For compound 6, on the other hand, two distinct regions of resonances can be identified as originating from the methyne and methyl protons (see Figure 1). The peaks are broad and paramagnetically shifted but still sufficiently sharp to provide useful structural information (this is not the case for the analogous Mo complex 2). Incidentally, the NMR spectrum also shows that the compound is not contaminated by significant amount of diamagnetic impurities. The relative intensities of the two regions at  7 -8 and -1 -4 allows their relative assignment to the methyne and methyl protons, respectively. The methyne region shows at least 9 peaks of various intensities, consistent with the population of several conformations and with the slow interconversion of these on the NMR time scale. At a first glance, it seems strange that the fast exchange limit for the isopropyl group rotation is observed for 1 and 5 while the slow exchange limit is observed for 6, since all system have similar four-legged piano stool geometries, thus the rotation barriers should be similar. In fact, the NMR observation of fast and slow exchange depends not only on the rate constant for the exchange process but also on the frequency difference between the resonances in the slow exchange limit. The much greater resonance spread for paramagnetic 6 is the likely cause for the observation of the slow exchange limit, even if the exchange rate may be similar to that in the diamagnetic compounds. It seems to us that this is the first report, for a 4 Cp derivative, where a restricted rotation of the isopropyl groups is directly evidenced. We are aware, however, of a similar situation for another paramagnetic complex, namely [ 4 CpNi(-Br)]2 [43]. In the solid state (see Figure 2) 4 exhibits a four-legged piano stool geometry with a ring centroid -metal distance of 2.05 Å, which is ca. 3% longer than the 1.99 Å found for [(C5H4 i Pr)WCl4] [24]. The W-Cl bonds of 4 (see (actual publication size: single column)

The crystal structure of 5 (Figure 3) reveals a molecule sitting on a crystallographic mirror plane which contains the tungsten-methyl and the C(3)-O(2) bonds as well as the ring CH unit and the ring center. The distance between the ring center and the tungsten atom (see table 3) is in the middle of the 2.00-2.02 Å range usually found for tricarbonyl(cyclopentadienyl)tungsten methyl complexes [44][45][46][47][48][49][50][51]. The W-CH3 distance is among the relatively short bonds within the 2.23-2.44 Å range reported in the literature. The methyl ligand is in an eclipsed position with regard to the unsubstituted ring carbon atom C(4), while the trans-CO ligand is accommodated between the two isopropyl groups turned away from each other on the opposite side of the ring. (actual publication size: single column)

Synthesis of 4 CpWCl4, 6 .

 46 Evaporation of the filtrate to dryness yielded 2.151 g (4.2 mmol, 83.3 %) of a yellow solid. IR (THF, cm -1 ): 2001.7 vs, 1907.9 vs.1 H NMR (C6D6, ): 4.73 (s, 1H, ring CH), 2.61 (septet, J = 6.8 Hz, 2H, CHMe2), 2.29 (septet, J = 7.2 Hz, 2H, CHMe2), 1.23 (d, J = 7.2 Hz, 6H, CH(CH3)2), 1.07 (d, J = 6.8 Hz, 6H, CH(CH3)2), 1.05 (d, J = 7.0 Hz, 6H, CH(CH3)2), 0.96 (d, J = 6.8 Hz, 6H, CH(CH3)2), 0.61 (s with satellites, JWH = 1.8 Hz, 3H, W-CH3).13 C NMR (C6D6, ): 233.6 (s with satellites, JWC = 66.18 Hz, 1C, W-CO), 220.4 (s with satellites, JWC = 77.21 Hz, 2C, W-CO), 117.2 (s, 2C, ring), 112.7 (s, 2C, ring), 91.3 (s, 1C, ring), 26.9 (s, i-Pr), 26.6 (s, i-Pr), 26.0 (s, i-Pr), 25.2 (s, i-Pr), 24.4 (s, i-Pr), 23.9 (s, i-Pr), -29.8 (s with satellites, JWC = 15.16 Hz, W-CH3). Anal. Calcd. for C21H32O3W: C, 48.89; H, 6.25. Found: C, 48.30; H, 6.10. Single crystals of this compound were grown from benzene at room temperature. Compound 5 (1.033 g, 2 mmol) was dissolved in 20 ml of dichloromethane. A solution of PhICl2 (1.375 g, 5 mmol) in 30 ml of dichloromethane was added dropwise during 15 min while stirring at room temperature. A strong gas evolution ensued and the color of the reaction mixture changed from brown to dark red-brown. The solution was stirred at room temperature overnight, during which time the color became green. A small amount (5 ml) of THF was added to destroy the excess of PhICl2. Then, the solution was evaporated to dryness leaving a dark-green solid (0.845 g, 1.5 mmol, yield: 75.6 %). Anal. Calcd. for C17H29WCl4: C, 36.52; H, 5.23. Found: C, 37.02; H, 4.83. The product is extremely soluble in dichloromethane, toluene and diethylether and could be recrystallized by cooling concentrated solutions in these three solvents to -70 °C. EPR (THF, r.t.): g = 1.958, width (peak-to-peak) = 13.5 G, aW = 64.2 G. 1 H NMR (at 200 MHz, CDCl3, ): nine signals of varying intensity between 7.15 and 7.85 (CHMe2, 1/2 ~ 20 Hz), four overlapping signals at 2.10, 1.55, 0.77, and 0.44 with 1/2 ca. 400-600 Hz. X-ray crystallography for compounds 4 and 5.

(

  Ring)WCl4 [Ring = V Cp, 4,4 Cp, 6] was carried out by returning to the original Schrock procedure via the tricarbonyl methyl intermediates (Ring)W(CO)3(CH3) [Ring = V Cp, 3,4 Cp, 5]. Compounds 3 and 5 show, like the hydride complex 1, the typical properties of this fourlegged piano stool half-sandwich class of complexes, notably the CO stretching frequencies are characteristic of this stoichiometry, as can be seen from the data in Table3. Conversion of 3 and 5 to the tetrachloro complexes 4 and 6 proceeded as expected according to Scheme 1.

Cp compounds 1 , 5

 15 and 6 shows interesting features concerning the mobility of the isopropyl substituents. For compounds 1 and 5, only two methyne peaks are observed, indicating that either only one rigid Cs conformer is present in solution (this could correspond to the structure observed in the solid state for compound 5, vide infra) or that the various possible conformers are rapidly equilibrating. Four Cssymmetric (2:2, A-D) and five C1-symmetric (of which four 3:1, E-H, and one 4:0, I) conformers are possible in principle (see Scheme 2) if we restrict the possibilities to having the methyne proton on the ring plane. The n:(4-n) nomenclature indicates the number of methyne H atoms that are oriented in the two opposite (clockwise and anti-clockwise) directions. All determined solid state structures, including the one reported in this work, show either a 2:2 or a 3:1 conformation. The structure of compound 5 (vide infra) corresponds to the arrangement A, like many other previously determined structures [2, 15, 33-39].

Figure 1 . 1 H

 11 Figure 1. 1 H NMR spectrum of compound (C5Pr i 4H)WCl4. The starred peak is due to a minor diamagnetic contaminant (probably silicone grease) (actual publication size: single column)

Figure 2 .

 2 Figure 2. ORTEP view of compound (C5Et5)WCl4.

Figure 3 .

 3 Figure 3. ORTEP view of compound (C5Pr i 4H)W(CO)3(CH3).

  R1 a = 0.026, wR2 b = 0.059 R1 a = 0.048, wR2 b = 0.109 R for IRC R1 a = 0.037, wR2 b = 0.064 R1 a = 0.055, wR2 b = 0.1127 Goodness-of-fit c 1.051 1.139 , max, min; e.Å -3 1.15 and -1.74 1.09 and -1.61 a R1=(||Fo|-|Fc||)/|Fo|. b wR2=[w(Fo 2 -Fc 2 ) 2 /[w(Fo 2 ) 2 ] 1/2 where w=1/[ 2 (Fo 2 +(0.00P) 2 +75.12P]for 4 and w=1/[ 2 (Fo 2 +(0.027P) 2 +1.47P]for 5 where P=(Max(Fo 2 ,0)+2*Fc 2 )/3. c Goodness of fit =[w(Fo 2 -Fc 2 ) 2 /(No-Nv)] 1/2 .

Table 2 ,

 2 average bond length 2.326 Å) are slightly shorter than those of the literature precedent (2.343 Å) and less uniform. Four chloro ligands arrange with five ethyl-substituted ring carbon atoms such that Cl(3) is fully eclipsed with one ring carbon atom and consequently displays the longest W-Cl bond (2.340 Å), whereas Cl(1) in trans position exhibits the shortest W-Cl distance of 2.289 Å.

Table 1 .

 1 Crystal data and structure refinement parameters for compounds 4 and 5.

	Compound	4 CpW(CH3)(CO)3(5)	V CpWCl4 (4)
	Formula	C21H32O3W	C15H25Cl4W
	M	516.3	531.00
	T; K	110(2)	110(2)
	Crystal system	Orthorhombic	Orthorhombic
	Space group	Pnma	Pbca (see text)
	a; Å	14.388(5)	14.2596(9)
	b; Å	16.962(5)	15.7854(9)
	c; Å	8.490(5)	16.0106(7)
	V; Å 3	2072.0(15)	3603.9(3)
	Z	4	8
	F(000)	1024	2056
	Dcalc; g/cm 3	1.655	1.957
	diffractometer	Nonius KappaCCD	Nonius KappaCCD
	scan type	 rot. and  scans	 rot. and  scans
	; Å	0.71073	0.71073
	; mm -1	5.591	6.992
	Crystal size; mm 3	0.37 x 0.25 x 0.12	0.15 x 0.12 x 0.12
	sin()/ max; Å -1	0.65	0.53
	Index ranges	h: -17; 18	h: -9; 15
		k: -21; 20	k: -16; 16
		l: -11; 9	l: -13; 17
	RC = Refl. Collected	8501	9119
	IRC = independent RC	2417 [R(int) = 0.0372]	2263 [R(int) =0.082]
	IRCGT = IRC and [I>2(I)]	1920	1896
	Refinement method	Full-matrix L.S. on F 2	Full-matrix L.S. on F 2
	Data / restraints / param. 2417 / 0 / 170	2263 / 0 / 106
	R for IRCGT

Table 3 .

 3 Comparison of CO stretching frequencies for substituted cyclopentadienyl complexes of tungsten.

	Compound		(CO)/cm -1	Solvent Ref.
	CpW(CO)3Me	2020 vs	1930 vs	CCl4	[52]
	4 CpW(CO)3Me, 5	2001.7 vs	1907.9 vs	THF	This work
		2010.1 vs	1919.2 vs	pentane
	V CpW(CO)3Me, 3	2008.3 s	1917.3 vs	pentane This work
	Cp*W(CO)3Me	2013 s	1920 vs	pentane [53]

Acknowledgement

We are grateful to the French Ministery of Research and to the CNRS for funding, and the French and German Ministeries of Foreign Affairs for a bilateral travel grant (Procope program).

Supplementary Material

Crystallographic data for the structural analysis have been deposited with the Cambridge


Thomas E Bitterwolf

W Bruce Scallorn

J Timothy Bays

Callie A Weiss

John C Linehan

James Franz

Rinaldo Poli

Photochemical intermediates of trans-Rh(CO

Keywords: Photolysis, Amine derivatives, DFT analysis

come   L'archive ouverte pluridisciplinaire

Introduction

The recent simultaneous publication of photochemical strategies for the carbonylation of ethane [1] and methane [2] using trans-Rh(CO)(PMe3)2Cl in supercritical CO2 (sc-CO2) attests to a continuing interest in the utilization of rhodium(I) photocatalysts to carry out fundamental chemical transformations on hydrocarbon substrates. Photoassisted carbonylation of C-H bonds in aromatic hydrocarbons and alkanes has been reported by Fisher and Eisenberg [3], Tanaka and coworkers [4], Browse and Goldman [5] and their coworkers. For benzene, Reaction (1) has been shown to be catalyzed by a number of metal carbonyl phosphine compounds including IrH3(CO)(dppe) [3a,3c], Ir(CO)(PPh3)2Cl [3b,3c], Rh(CO)(PPh3)2Cl [3b,3c], Rh(CO)(PPh3)2H [3b,3c], Rh(CO)(PMe3)2Cl [4,5] and Ru(CO)4(PPh3) [3d].

R-H + CO  R-CHO

Reaction (1) In the absence of CO, trans-Rh(CO)(PMe3)2Cl (1), catalyzes the thermodynamically unfavorable dehydration of alkanes. Goldman and coworkers [6] have examined the mechanism of this reaction and developed convincing evidence that the photolysis (λirr = 3669±20 nm) results

in CO-loss to form [Rh(PMe3)3Cl], proposed to be the active intermediate in the dehydrogenation process.

Detailed mechanistic studies by Ford and co-workers [7] and Rosini et al. [8] have established that there may be two pathways for these carbonylation reactions. One pathway involves loss of CO forming [Rh(PMe3)2Cl] that can undergo C-H insertion and subsequent CO recapture to yield a hexacoordinate species of the general form Rh(CO)(H)(R)(PMe3)2Cl. A second pathway appears to involve direct reaction of substrate with an excited state of I to yield the same intermediate. A secondary photochemical step initiates loss of RCHO from the hexacoordinate intermediate. There is a marked wavelength dependence for the two oxidative addition pathways with CO-loss requiring lower energies. At a photolysis wavelength of 355 nm both pathways appear to be operative although the partitioning between the two pathways is not known. 4b] have reported a wavelength dependent product distribution in the photolysis of 1 with toluene and decane. High-energy light (λirr > 295 nm) results primarily in meta and para substitution of toluene and terminal carbonylation of decane. In contrast, lower energy photolysis (λirr > 375 nm) results in substantial methyl carbonylation of toluene and internal carbonylation of decane. It is not presently known whether the difference in product distribution is in some way related to the two oxidative addition pathways noted above.

Our long-standing interest in the identification of photochemical intermediates in frozen organic glasses prompted us to initiate what was expected to be a short investigation into the photochemistry of 1. The results of what ultimately proved to be a multi-year odyssey are reported in this paper.

Results

Experimental procedures for photochemical studies in frozen organic glasses have been previously reported [9]. A variety of optical filters were used to control the incident light during photochemical experiments. Compounds 1 [8], trans-Rh(CO)(PBu3)2Cl (2) [10], and cis-

Rh(CO)2(i-Pr2HN)Cl (3) [11],
were prepared by standard literature methods. trans-Rh(CO)(i-Pr2HN)2Cl (4), was prepared by photolysis of 3 in heptane with an excess of i-Pr2HN. Details of this synthesis will be reported in a subsequent publication [12].

Compound 1 is only moderately soluble in Nujol at room temperature. Samples were prepared by grinding a few crystals of 1 with Nujol followed by lengthy centrifugation to separate fine crystallites. IR spectra of the resulting solutions both at room temperature and at ca. Photolysis of Nujol glass solutions of 1 at λirr > 450 nm resulted in no detectable changes in the IR spectrum. Photolysis at λirr = 4009±70 nm resulted in a decrease in the intensity of the band of 1 at 1964 cm -1 and growth of a new band at 1955 cm -1 . Upon extended photolysis (λirr ≥ 400 nm, 6 h) a low, broad band may be discerned at about 1938 cm -1 (Fig. 2a). No band for 'free' CO (21319±2 cm -1 ) is observed at these photolysis wavelengths even after very long photolyses.

Annealing a sample photolyzed in this manner to 153 K resulted in decrease of the 1955 cm -1 band, growth of the band at 1964 cm -1 and small growth in the band at 1938 cm -1 (Fig. 2b). Photolysis of a fresh sample of 1 (330 < λirr < 400 nm, 1 h) results in a decrease in the carbonyl band of 1 and growth of bands at 2131, 2012, 1954, and 1938 cm -1 (Fig. 3a). Photolysis of a fresh sample of 1 (λirr = 280±10 nm, 30 min) resulted in decrease in the band of 1 and growth of bands at 2131 and 2012 cm -1 (Fig. 4a). Back photolysis of this sample (λirr =450±70 nm, 10 min) resulted in a bleaching of the band at 2012 cm -1 and growth of a band at 1962 cm -1 presumed to be 1, with a low energy shoulder at 1955 cm -1 (Fig. 4b). The band at 1955 cm -1 may be a result of direct photolysis of 1 at this wavelength and not be related to the back photolysis of the 2012 cm -1 band. Annealing studies of a sample of 1 that had been photolyzed for an extended time (λirr =/280±10 nm, 4.5 h) were particularly informative. As before, photolysis resulted in a decrease in the band of 1 and growth of bands at 2131 and 2012 cm -1 (Fig. 5a). Annealing this sample to 130 K resulted in a decrease in the band at 2131 and a previously unseen band at 1955 cm -1 and growth of bands at 2012, 1964, and 1938 cm -1 (Fig. 5b). Further annealing to 170 K resulted in further decrease in the bands at 2131 and 1955 cm -1 as well as a decrease in the band at 2012 cm -1 and growth of the bands at 1964 and 1938 cm -1 (Fig. 5c). To clarify the identity of the new species, 13 CO labelled 1 (about 90% enrichment that also contains a trace of 13 C 18 O) was prepared and photolyzed (330 < λirr < 400 nm, 15 min) (Fig. 3b).

Bands of 1 (1962 cm -1 ), 1-13 CO (1916 cm -1 ), and 1-13 C 18 O (1873 cm -1 ) were found to decrease while new bands at 2082,2011,1968,1954,1908, and 1865 cm -1 were found to grow in. The observed bands at 1908 and 1865 cm -1 correspond to the 13 CO and 13 C 18 O labeled analogues of the species at 1955 cm -1 , while the band at 1968 cm -1 corresponds to the 13 CO labeled analogue of the previously observed band at 2012 cm -1 . The band at 2082 cm -1 is 'free' 13 CO in the Nujol glass matrix. Free 12 CO and 13 C 18 O were not detectable above the background noise.

In 2-methyltetrahydrofuran glass at ca. 90 K, 1 has a slightly broadened band at 1961 cm -1 .

Photolysis of this band at energies as low as 500 nm resulted in a decrease in the bands of 1 and growth of a band at 1952 cm -1 . Higher energy photolysis resulted in CO loss, but no new bands were observed. In an effort to clarify the identities of the various species present in the photolysis of 1 an effort was made to prepare and isolate cis-Rh(CO)2(PMe3)Cl. Trimethylphosphine was slowly added to a solution of Rh2(CO)4Cl2 in petroleum ether under CO as described by Poliblanc and co-workers [13] with the formation of Rh(CO)2(PMe3)Cl being confirmed by the appearance of carbonyl bands at 2093 and 2001 cm -1 . A CO saturated solution was stored at -78 °C for several days during which time cherry red crystals appeared. All attempts to isolate these crystals resulted in a rapid color change from bright red to dull tan. The resulting tan product was shown by IR to be Rh2(CO)2(PMe3)2(µ-Cl)2. We believe that the red crystals are Rh(CO)2(PMe3)Cl and attribute their color to the well known d-d stacking effects observed for some Rh(CO)2(ammine)Cl derivatives [11,14]. When petroleum ether solutions containing Rh(CO)2(PMe3)Cl were frozen to liquid nitrogen temperatures a purple color was observed that we believe to be due to continued growth of d-d stacks at low temperature.

Discussion

The lowest energy photoproduct, A, of 1 and 2 forms upon photolysis into the bottom edge of the 360 nm band without loss of CO. In our preliminary analysis of these spectra we speculated that it might arise from loss of a PMe3 ligand as has been reported by Perutz and coworkers [15] for cis-Ru(PMe3)4H2. Reinforcing this hypothesis was the similarity of the IR band of [Rh(CO)(PMe3)3]Cl with that of C.

Our attempts to prepare and isolate Rh(CO)2(PMe3)Cl as a precursor to the putative [Rh(CO)(PMe3)Cl] intermediate proved unsuccessful, thus we opted to prepare a corresponding ammine series of derivatives to serve as analogues. The choice of ammine in this case was dictated by our desire for a comparatively simple ligand, but also one that would preclude d-d

stacking of the square-planar rhodium molecules. Our first choice for a model compound was the previously unknown Rh(CO)2(Et3N)Cl. While this compound was easily formed upon addition of Et3N to Rh2(CO)4(µ-Cl)2, it proved to be exceptionally air-sensitive giving rise to a blue species, we believe to be an O2 adduct. It appears that there has only been one other report of an attempted synthesis of Rh(CO)2LCl derivatives with tertiary amines and these compounds were simply described as being unstable [16]. Although we were ultimately able to carry out Nujol matrix studies on Rh(CO)2(NEt3)Cl that parallel the results for the di(isopropyl)amine derivative described below, the quality of these spectra were not high and it was clear that impurities were present.

As an alternative to Rh(CO)2(Et3N)Cl, the known, air-stable derivative Rh(CO)2(i- Photolysis of 3 is expected to give rise to two isomeric carbonyl loss products (Fig. 7) as is observed. Chloride is a potential π-electron donor while there can be no comparable donation from the ammine ligand. Accordingly, we have assigned the photoproduct with the lower carbonyl stretching frequency (1960 cm -1 ) to the isomer with the carbonyl trans to the chloride, and the species with the higher carbonyl stretching frequency (1977 cm -1 ) to the isomer with the carbonyl trans to the ammine. 22 cm -1 above that of its ammine analogue (1960 cm -1 ) or about 20 cm -1 above that of 1. DFT calculations on 1 and [Rh(CO)(PMe3)Cl] predict the monophosphine species to have a carbonyl stretching frequency 29 cm -1 above that of 1. This is remarkably consistent with our prediction above. We note that [Rh(CO)(i-Pr2HN)Cl] has a carbonyl stretching frequency 20 cm -1 above that of Rh(CO)(i-Pr2HN)2Cl. These data are summarized in Fig. 8. Based on this analysis, we must conclude that the photoproduct A cannot be assigned to the phosphine loss species , where R = alkyl and aryl groups [17]. It was noted that electronic transitions at 360 nm sharpened when observed at 77 K, but the reported measurements did not extend below 300 nm, thus the 275 nm feature observed in the current work was not seen. Brady et al. attribute the 360 nm transition to an a1(z 2 )-b1π (HOMO-LUMO) transition with MLCT character. The second highest excitation would be a1(z 2 )-a1(x 2y 2 ). Since the latter orbital is antibonding with respect to the ligands it may well be associated with the higher energy CO-loss excitation. The electronic spectra of the C2v M(CO)L2Cl species parallel to those observed for D4h [M(PC2P)2] +1 and [M(PC=CP)2] +1 complexes and are typically interpreted by analogy to the more symmetric case (Fig. 9). Geoffroy et al. [18] and Fordyce and

Pr2HN
Crosby [19] have noted that the lower energy transition of the complexes with D4h symmetry is 1 A1g-3 A2u (a1g-a2u), while the next higher energy band is 1 A1g-1 A2u (a1g-a2u). It is interesting to note that large Stokes shifts observed in the emission spectra of Rh(I) D4h complexes have been interpreted to suggest substantial distortion of the excited states. Geoffroy et al. [18] propose that the emitting state is not purely MLCT in character, but includes one or more d-d states leading to a tetrahedral distortion. We shall return to this point below.

Fig. 9. Molecular orbital energy level diagram (ref. [17]).

As noted above, spectral studies on Rh(I) complexes have suggested that the lowest energy electronic transition is consistent with a triplet excited state, and that some evidence exists to suggest that this excited state deviates from square-planar geometry. We have carried out density functional theory (DFT) calculations of 1 and its PH3 model compound in an attempt to better understand the nature of this excited state. Calculated bond lengths and angles for Rh(CO)(PH3)2Cl and 1 are presented in Tables 1 and2 along with crystallographically derived values for 1. The calculated structures are presented in Fig. 10. Calculated energies and CO stretching frequencies are presented in Table 3.

Rh(CO)(PH3)2Cl was found to have two excited state, triplet geometries: a distorted-planar geometry that lies 36.4 kcal mol -1 above the ground state and a nonplanar geometry in which the chloride ligand has moved out of the molecular plane that is 30.8 kcal mol -1 above the ground state. The geometries of these triplet species are presented in Fig. 10. These very surprising results prompted us to carry out calculations on 1 itself, for which we found a non-planar triplet excited state 35.3 kcal mol -1 above the ground state. Like the PH3 case, the chloride ligand was found to have moved out of the molecular plane. Bond lengths and angles of the three triplet species are given in Table 1. Calculations of the IR frequencies of both the ground and excited states were carried out and these values are presented in Table 3. The calculated ground state vibrational frequency is 1912 cm -1 and the frequency of the triplet species is calculated to be about 1.5 cm -1 below this. DFT calculated absolute values of vibrational frequencies for metal complexes are often observed to be shifted to lower energy and calculations on excited state species are particularly difficult. For this reason we note that calculations lend cautious support to the assignment of the species observed at 1955 cm -1 to a non-squareplanar triplet which is stabilized by the low temperatures and relatively inert matrix. This is consistent with the ease of conversion of this species to the starting material upon annealing. The data from both photolysis and annealing experiments suggest that C either arises as a secondary photolysis product of A or through warming of A. C is unaffected by annealing to as high as 170 K. The identity of this species is an enigma to us as an oxidative addition reaction between the metal and Nujol or a PMe3 ligand would yield a Rh(III) species that should exhibit higher carbonyl frequencies, not lower frequencies. The dilution of 1 in the Nujol matrices is sufficiently high as such we do not believe that intermolecular interactions between rhodium molecules can account for the observed bands.

High energy photolysis of 1 gives rise to CO-loss as well as to the appearance of a new species, B, with a carbonyl band at 2012 cm -1 . B is transformed to 1 by back photolysis. It is significant that B also appears upon annealing of a matrix that had been photolyzed at high energy and thus contained both free CO and the IR silent Rh(PMe3)2Cl species. No such species is observed for 2 or for 4.

Recapture of a CO by Rh(PMe3)2Cl results in reformation of 1, but could also give rise to the cis isomer of this compound. The cis isomer would be expected to be thermally and photochemically unstable and revert to the trans isomer as is observed. Failure to observe comparable bands in the photolysis of 2 may be due to the steric bulk of the tributylphosphine ligands making a cis isomer highly unstable. At very high energies it appears that B can form directly from 1, possibly through a tetrahedral excited state.

As noted in Section 1, has used time-resolved IR to study the photolysis of 1 in benzene. Two bands observed at 2130 and 2018 cm -1 in C6H6 were assigned to metal hydride bands. These bands were absent when the photolyses were carried out in C6D6. We suggest that the band observed at 2130 cm -1 may be attributable to free CO and it is likely that the 2018 cm -1 band is due to the same photoproduct observed in these matrix studies. Indeed, we have observed a species at 2019 cm -1 in a time-resolved IR study, the photolysis of 1 in supercritical CO2 [20]. The band at 2019 cm -1 decays slightly within the 150 ms time frame of the experiment. We must regard these interpretations as preliminary as we have no explanation for why these bands were not observed in the deuterated solvent.

To further clarify the identity of the 2012 cm -1 species we have carried out DFT calculations on cis-Rh(CO)(PMe3)2Cl and found that this isomer is expected to have a carbonyl stretching frequency 41 cm -1 higher than that of the trans isomer. This yields a predicted value of 2003 cm -1 that is in good agreement with the observed values of 2012-2018 cm -1 . Representative calculated bond lengths and angles for the cis isomer are presented in Table 2. The difference in carbonyl stretching frequency between the cis and trans isomers may be readily attributed to the carbonyl group being trans to a π-donor chloride in 1 and trans to a weak π-acid trimethylphosphine in the cis isomer. Finally, it is important to note that the energy range necessary for direct trans to cis conversion of 1 corresponds to that described by Rosini et al. [8] as being associated with oxidative addition of benzene to the intact excited state of 1. In both cases these processes would appear to be triggered by photolysis into the electronic band with a maxima at 275 nm.

Table 3. Calculated and experimental bond lengths (Å) and angles (°) for 1, its non-planar triplet, cis isomer and PMe3 loss species, Rh(CO)(PMe3)Cl R = H R = CH3 E (kcal mol -1 ) νCO (cm -1 ) E (kcal mol -1 ) νCO (cm -1 ) (cal) νCO (cm

Experimental

Compounds 1 [8], 2 [10], and 3 [11] were prepared by standard literature procedures. The details of the preparation of 4 and other bis(ammine) derivatives will be presented elsewhere [12]. Electronic spectra were recorded on a Cary 2200 Spectrometer. IR spectra were recorded using a Perkin-Elmer Spectrum 1000 FTIR Spectrometer. IR spectra were recorded at 4 cm -1 resolution. Nujol matrix studies were carried out using a cryostat designed by Dr. A. Rest of the University of Southampton. 2-Methyltetrahydrofuran matrix studies were carried out using a Graseby Specac Variable Temperature Cryostat. Photolyses were carried out using a 350 W high pressure Hg lamp. Optical filters were used to control the wavelength ranges of the incident radiation.

Computational details

All systems have been studied using the B3LYP hybrid density functional theory [21],

which has proven to be very reliable for a broad range of organometallic systems similar to those reported here [22]. All geometry optimizations and frequency calculations were carried out with the GAUSSIAN 98 program package [START_REF] Frisch | Gaussian 98[END_REF] using the standard LanL2DZ basis set, which includes both Dunning and Hay's D95 sets for H and C [START_REF] Dunning | Modern Theoretical Chemistry[END_REF] and the relativistic core potential sets of Hay and Wadt for the heavy atoms [25]. The input geometry for the singlet trans-Rh(CO)(PH3)2Cl

and trans-Rh(CO)(PMe3)2Cl molecules were adapted from the published X-ray structure of the PMe3 complex [26]. The corresponding triplet structures were obtained starting from a pseudotetrahedral input geometry with bond distances set to those obtained from the corresponding singlet optimization. A different, higher energy local minimum having a planar geometry was obtained for the PH3 model compound when starting from the optimized planar geometry of the singlet state. For the larger PMe3 molecule, this calculation was not carried out.

All calculations were carried out without imposed symmetry. Spin contamination was carefully monitored and the value of <S2> for the unrestricted B3LYP calculations on the triplet states at convergence [2.0076 for RhCl(CO)(PH3)2; 2.0083 for RhCl(CO)(PMe3)2] indicated minor spin contamination. The energies reported in Sections 2 and 3 do not include a correction for zeropoint energy.

  90 K had single, sharp lines for the carbonyl stretching band of 1 characteristic of Nujol solutions and not the broad bands typical of mulls. Nujol glass solutions of 1 at ca. 90 K are found to have two absorption bands at 360 and 275 nm (Fig.1). At room temperature these bands are considerably broadened. Neither IR nor UV-vis spectra contained any bands that might be attributed to d-d stacked dimers or oligomers.

Fig. 1 .

 1 Fig. 1. Electronic spectrum of 1 in Nujol at ca. 90 K. The small features at 475 and 650 nm are instrument artifacts.

Fig. 2 .

 2 Fig. 2. (a) 6 h photolysis of 1, λirr > 400 nm. (b) Anneal of sample to 153 K.

Fig. 3 .

 3 Fig. 3. (a) 1 h photolysis of 1, 330 < λirr </400 nm. (b) 15 min photolysis of 13 CO labeled 1,

Fig. 4 .

 4 Fig. 4. (a) 30 min photolysis of 1, λirr =/280±10 nm. (b) 10 min back photolysis, λirr

Fig. 5 .

 5 Fig. 5. (a) 4.5 h photolysis of 1, λirr =/280±10 nm. (b) Anneal to 130 K. (c) Anneal to 170

  Photolysis (350 < λirr < 400 nm, 30 min) of a frozen Nujol glass sample of 2 resulted in a decrease in the band of 2 (1954 cm -1 ) and growth of a new band at 1947 cm -1 . No free CO was

  )Cl (3), was prepared. Compound 3 is known to exist in the solid state and in solution as a monomeric species [11]. IR bands of 3 are observed in Nujol at 2080 and 2000 cm -1 , somewhat lower in energy than Rh(CO)2(PMe3)Cl in CO saturated petroleum ether for which bands are observed at 2094 and 2001 cm -1 .

Fig. 7 .

 7 Fig. 7. CO-loss isomeric photoproducts of Rh(CO)2(ammine)Cl derivatives.

[

  Rh(CO)(PMe3)Cl].

Fig. 8 .

 8 Fig. 8. Summary of observed and calculated rhodium carbonyl phosphine and ammine derivatives.

Fig. 10 .

 10 Fig. 10. Computed geometries and energies for Rh(CO)(PH3)2Cl, Rh(CO)(PMe3)2Cl and their related planar and non-planar triplet excited states.

Table 1

 1

	. Calculated bond lengths (Å) and angles (°) for Rh(CO)(PH3)2Cl
		Singlet	Triplet, non-planar Triplet, planar
	Bond lengths
	Rh-P	2.393	2.537	2.735
	Rh-Cl	2.447	2.515	2.514
	Rh-C	1.842	1.925	1.935
	C-O	1.187	1.181	1.179
	Bond angles

Table 2 .

 2 Calculated and experimental bond lengths (Å) and angles (°) for 1, its non-planar triplet, Solution photochemical studies of 1 by Ford and coworkers [7] and Rosini et al.[8] have provided evidence for the existence of two photochemical pathways in the oxidative addition of hydrocarbons, one involving CO loss and a second involving direct interaction of an excited state of 1 with hydrocarbon. Our matrix studies now provide support for the existence of three distinct

	cis isomer and PMe3 loss species, Rh(CO)(PMe3)Cl
		1, cal.	1, exp. Ref.	Non-planar	cis-Rh(CO)(PMe3)Cl	Rh(CO)(PMe3)Cl
			[26]	triplet
	Bond lengths
	Rh-P	2.412	2.308	2.512	2.38 (cis-CO)	2.3887
					2.498 (trans-CO)
	Rh-Cl	2.471	2.354	2.556	2.4588	2.3887
	Rh-C	1.829	1.770	1.920	1.8592	1.8444
	C-O	1.191	1.146	1.185	1.1822	1.1876
	Bond angles
	Cl-Rh-P 85.66	88.7	87.30	180.03
	Cl-Rh-C 179.98 179.4	106.28	87.59	177.63
	P-Rh-P	171.31 177.2	155.21	97.90
	P-Rh-C 94.34	91.3	102.37	180.04 (trans )
					92.14 (cis )
	Rh-C-O 179.97 178.4	179.51	179.56	179.49
	4. Conclusions

Acknowledgements

T.E.B. wishes to thank the Research Corporation for their generous award of a Research Opportunity grant and the Pacific Northwest National Laboratory for financial support. We thank Professor Goldman for his generous gift of our first samples of 1, and Dr. G. Rosini, Professor Goldman and Professor Ford for helpful advice. We sincerely thank a referee for pointing out the probable direct formation of B from 1 at high energy.


Philippe Swartvagher

Interferences between Communications and Computations in Distributed HPC Systems

come   L'archive ouverte pluridisciplinaire

Interferences between Communications and Computations in Distributed HPC Systems

Philippe S -Alexandre D , Emmanuel J TADaaM -INRIA, LaBRI, France Distributed HPC systems HPC: High-Performance Computing Distributed: many computers connected through a network Common usecases: scientific computations, simulations (physics, chemistery, weather, astronomy, ...) Goal: get the highest computing performance One HPC cluster is composed of: Several nodes... ... composed of several computing units (CPUs, GPUs, ...)... ... composed of many cores.

Computations and communications in parallel

While many cores are computing, a core makes communications with other nodes A technic known for improving performances

Interferences between communications and computations ?

Side-by-side communications and computations share common resources (memory bus) Impact of memory contention on computations already observed in the litterature Can simultaneous communications and computations have a negative impact on each other ? Is there an impact on communications ? Which factors change the impact of interferences ?

Methodology

Goal: compare performances of communications and computations executed alone or simultaneously Own benchmark with the following steps: . Computations alone . Communications alone . Computations with communications in parallel Parallelization with OpenMP, communications with MPI

Impact of memory contention

Extreme case: computing cores executing memory-bound kernels

The higher the number of computing cores, the higher the memory contention and the more communication performances lower

Impact of the size of transmitted data

Does the size of transmitted data across the network affect the memory contention ?

The bigger the communicated data, the more it disturbs computations, and the more communications are disturbed by computations

Impact of arithmetic intensity

Arithmetic intensity: number of arithmetic operations per accessed byte of memory Computing cores execute instructions with tunable arithmetic intensity Interferences disappear when computations become CPU-bound

Conclusion

Computations can slow down communications it depends on arithmetic intensity of computations it depends on size of transmitted data vice-versa, communications can impact computations too Future works:

Better understand origins of interferences to model and predict them Consider data movements between main memory and GPUs Take into account these interferences in scheduling made by runtime systems https://www.labri.fr/ Thursday May 20, 2021 Twitter: @labriOfficial


Edmond Collange

Juan A Garcia

Rinaldo Poli
email: rinaldo.poli@u-bourgogne.fr

Prof Rinaldo Poli

Nature of (C5Me5)2Mo2O5 in Water-Methanol at pH 0-14. On the Existence of (C5Me5)MoO2(OH) and (C5Me5)MoO2 + : a Stopped-Flow Kinetic Analysis

Abstract.

A stopped-flow analysis of compound Cp*2Mo2O5 (Cp* =  5 -C5Me5) in 20% MeOH-H2O in the pH range 0-14 has provided the speciation of this molecule as well as the rate and mechanism of interconversion between the various species that are present in solution. The compound is a strong electrolyte in this solvent combination, producing the Cp*MoO2 + and Cp*MoO3 -ions in equilibrium with a small amount of Cp*MoO2(OH), the latter attaining ca. 15% maximum relative amount at pH 4. At low pH (< 2.5) Cp*MoO2 + is essentially the only species present in solution, while the anion Cp*MoO3 -is the dominant species at pH > 6. The acid dissociation constant of Cp*MoO2(OH) has been measured directly (pK = 3.65±0.02) while the pK for the protonation equilibrium leading to Cp*MoO3H2 + is estimated as < 0. The three trioxygenated species establish rapid proton transfer equilibria among themselves, but transform to the dioxo species Cp*MoO2 + by two slower and independent first order pathways: loss of H2O from Cp*MoO3H2 + and loss of OH -from Cp*MoO2(OH). The former pathway is prevalent at pH < 2, where the transformation proceeds to completion, giving rise to kinetics that are first order in metal and first order in [H + ]. The reverse process is quantitative at pH > 5. The prevalent pathway at high pH is the addition of OH -to Cp*MoO2 + , giving rise to kinetics that are first order in metal and first order in [OH -]. The kinetics of the equilibration process at intermediate pH are affected by the buffer concentration, indicating a general acid/base catalytic phenomenon.

The complete elucidation of the kinetic and thermodynamic scheme was made possible by the combined analyses of the equilibrium in the 3-5 pH range and the kinetics in the extreme pH regions.

Introduction.

High oxidation state organometallic chemistry has experienced a rapid development in the last 20 years, mostly justified by the search for efficient oxidation catalysts. This is exemplified by the numerous studies carried out on the half-sandwich oxo derivatives of rhenium. [1][2][3] The coordination sphere of high oxidation state metals typically involves electronegative and -donating ligands such as the halides or negatively charged oxygenbased (oxo, alkoxo) or nitrogen-based (nitrido, imido, amido) ligands. These conditions confer a high degree of covalency to any metal-carbon bond, which consequently becomes quite resistant to hydrolytic conditions. The cyclopentadienyl ligands are perhaps the most compatible organic fragments in this area of organometallic chemistry. It is therefore somewhat surprising that the physical behavior and chemical reactivity of high oxidation state organometallics is not systematically investigated in water, although some of these systems are synthesized in water or by the use of aqueous reagents. Some of the earliest examples of high oxidation state organometallics are the molybdenum derivatives of general formula (Ring)2Mo2O5 (Ring = substituted cyclopentadienyl ligand), first developed in the laboratory of M. L. H. Green for the parent cyclopentadienyl system. [4][5][6] We have recently reported improved syntheses of the Cp and Cp* compounds (Cp =  5 -C5H5; Cp* =  5 -C5Me5), and extended them to the preparation of new derivatives containing the sterically congested C5HPr i 4 and 1,2,4-C5H2Bu t 3 rings. 7 The structures of these compounds display neutral dinuclear units with symmetric Mo-(-O)-Mo bridges, like the various X-ray structures previously reported for different polymorphs of the Cp* compound. [8][9][10][11] A related complex, Cp*MoO3 -, has been reported independently by Geoffroy and Sundermeyer. [12][13][14] These complexes can indeed be prepared in an aqueous medium, 4,7,[13][14][15] but most reported investigations have been restricted to the use of non aqueous solvents. [8][9][10][11][16][17][18][19] We have initiated a research effort aimed at increasing our basic knowledge of the physical properties and chemical reactivity of organomolybdenum compounds in a variety of high oxidation states in water, with the goal in mind of developing new aqueous organometallic chemistry, catalysis and electrocatalysis. In this contribution, we report a thorough study of the Cp*Mo VI system in an aqueous environment in the entire pH range (0-14). Since compound Cp*2Mo2O5 is insoluble in pure water, while alkali metal salts of Cp*MoO3 -(e.g. sodium) are lipophobic, we were forced to carry out our investigation in a mixed MeOH-H2O solvent. However, the use of just 20% MeOH proved sufficient to keep the neutral compound in solution at sufficient concentrations (up to 4 . 10 -4 M) to carry out our UV-visible spectroscopic investigations, while it does not interfere with the aqueous chemistry of the Cp*Mo VI species as shown by the results obtained.

Experimental Section.

Reagents and solutions. All chemicals were of reagent grade and used as supplied without further purification. HNO3 and NaOH standards solutions were from Merck-Prolabo normadoses. The buffers were constituted in single conjugate pairs or in Britton-Robinson (B.R.) mixtures. 20,[START_REF] The | buffers were solutions of acetic, orthophosphoric, or boric acid partially neutralized with NaOH and containing NaNO[END_REF] The ionic strength of all solutions was adjusted to 0.1 M by the addition of NaNO3. Compounds Cp*2Mo2O5 and Cp*MoO2Cl were prepared according to the literature procedures. 7 pH and conductivity measurements. The pH of the buffer solutions was determined, at 25 °C, with a Methrom 702 SM pHmeter. The electromotive force of the cell is expressed by the equation E = Eo -k(pH) + Ej. [START_REF] Costa | [END_REF] In the presence of supporting electrolyte, such as 0.1 M NaNO3, the junction potential Ej is generally considered as constant in the pH range 2-12. [START_REF] Bates | Determination of pH: theory and practice[END_REF][START_REF] Bosch | [END_REF][25] The above equation gives pH = (Eo-E)/k + Ej/k = pHread + pH. The slope k ( 59 mV) was obtained by calibration at 25 °C with NBS standard buffers prepared in aqueous medium (0.05 M potassium biphthalate, pH = 4.008; 0.05 M borax, pH 9.196). 26 The correction term, pH, was determined daily by NaOH titration of a HNO3 solution. It is the difference between the calculated and read pH values obtained in the acidic range. This term was generally close to -0.15 pH units. The pKs of the mixed solvent (80:20 H2O-MeOH) is the sum of the experimental pH and the calculated pOH values in the basic range. 27 We have obtained pKs = 14.250.02, close to the value determined by Rochester. 25 The electrical conductivity measurements were carried out This spectrum is qualitatively identical but has a lower intensity than that recorded on the same solution after lowering the pH to 1 by the addition of a strong acid (HNO3), see trace b of Figure 1. The reason of this phenomenon will become apparent after we have examined in detail the kinetics in the entire pH range (section d). When NaOH is added to the starting solution, on the other hand, this becames essentially colorless, see trace c of Figure 1. The latter change is attributed to the formation of the previously described anion Cp*MoO3 -. [12][13][14] This solution turns back yellow if, at this point, the pH is lowered again.

The kinetics of this acidification process will be examined in detail in the next section. All spectral changes witnessed during this investigation as a function of pH are perfectly reversible with no loss of intensity, indicating that all transformations are analytically clean.

<insert Figure 1> The anation of Cp*2Mo2O5 has been investigated by stopped-flow kinetics with UVvisible detection in the basic pH range by using NaOH solutions at different concentrations (pH 11.42-13.25) or buffers at pH < 11. In all cases, the final spectrum is identical to that shown in Figure 1 (c), showing that the transformation into the anionic complex Cp*MoO3 - proceeds to completion under these conditions. All tranformations obey precise first order kinetics and the log of the observed rate constant (k+obs) varies linearly with the pH (slope 1) for the pH > 11 data, as shown in Figure 2. This demonstrates that the slow step has a first order dependence on the concentration of OH -. All k+obs values can be found in the Supporting Information. The behavior at pH < 11 will be analyzed in section h.

<insert Figure 2> The rate law of this transformation could in principle be easily interpreted on the basis of a rate determining attack of OH -to the dinuclear Cp*2Mo2O5 complex, the nucleofuge being the Cp*MoO3 -anion. The resulting product of nucleophilic exchange, the conjugate acid Cp*MoO2(OH), would be immediately deprotonated under these pH conditions. The pK study of the Cp*MoO2(OH)/Cp*MoO3 -couple, shown in the next section, confirms the validity of this statement. It will be shown later, however, that the given mechanistic interpretation is incorrect.

(b) Protonation of Cp*MoO3 -: Formation of Cp*MoO2(OH) and pK Study.

A stock solution of Cp*MoO3 -in 20% MeOH-H2O was prepared by adding the minimum excess amount of NaOH to a Cp*2Mo2O5 solution prepared as described above, in order to adjust the pH around 8-9. This was then mixed in the stopped-flow apparatus with various strong acid or buffer solutions, obtaining a final acidic pH. The UV-visible monitoring indicated the immediate change of the spectrum when the final pH was < 6, followed by a slower spectral evolution. We interpret this behavior as the result of an immediate protonation yielding Cp*MoO2(OH) (proton transfer reactions are generally diffusion-controlled), which then evolves further. The implication of Cp*MoO2(OH) during the protonation of Cp*MoO3 -, ultimately leading to the dinuclear complex Cp*2Mo2O5, has already been proposed but no spectroscopic evidence for the existence of this compound was reported. 14 Reactions of Cp*MoO3 -with other electrophiles Y + , on the other hand, have allowed the isolation of stable Cp*MoO2(OY) complexes, for instance with Y = Si(CH2Ph)3. 19 The same transformation occurs for the tungsten analogue.

The pK value for the acid dissociation of Cp*MoO2(OH) has been obtained from the analysis of the first spectrum (recorded within 1 ms from the time of mixing) as a function of pH, see Figure 3. The solutions buffered at pH > 6 showed essentially the unaltered spectrum of Cp*MoO3 -, whereas the solutions buffered at pH < 2 showed essentially the same spectrum, attributed to Cp*MoO2(OH). A SPECFIT [START_REF] Binstead | Specfit/32[END_REF] global analysis yields a pK = 3.65±0.02 for the acid dissociation constant.

<insert Figure 3> (c) Kinetic analysis of the Cp*MoO2(OH) evolution at pH  2.

After the quantitative transformation of Cp*MoO3 -to its conjugate acid in the stopped-flow apparatus, the spectrum evolved ultimately yielding a pH independent final spectrum (at pH < 2), showing a quantitative transformation under these conditions. Figure 4 shows an example at pH 1. It is to be noted that this final spectrum (trace b) corresponds to trace b of Figure 1 and not to the spectrum of the starting Cp*2Mo2O5 solution (trace a of Figure 1). Thus, the final product obtained from Cp*MoO2(OH) under these conditions is not the dinuclear species.

<insert Figure 4> All kinetics are first order in metal and give rise to the observed rate constants k-obs reported in the Supporting Information. These rate constants depend on the pH as shown in Figure 2 This solution evolves to the same equilibrium solution (trace c), following again first order kinetics. At each investigated pH value, starting from either solution yielded the same equilibrium spectrum with identical observed first order rate constants (within experimental error), provided that the same buffer concentration was used for both experiments, see section h. All individual values of the observed rate constants (kobs) can be found in the Supporting Information.

<insert Figure 5> The results of the kinetics investigations in the equilibrium region demonstrate that compound Cp*2Mo2O5 does not maintain its molecular, dinuclear nature in the 20% MeOH-H2O solution. Under the assumption that this species is indeed present and transforms to the anion by a first order process as seen in section (a), the microscopic reverse which becomes observable in the equilibrium region should have a second order dependence on the metal concentration. This situation occurs, for instance, for the equilibration between the mononuclear HMoO3 + (aq) and the dinuclear H2Mo2O6 2+ (aq). [START_REF] Ojo | [END_REF] However, this is not the case for our Cp*Mo VI system. This is further confirmed by the little or no dependence of the measured rate on the metal concentrations for the same pH value (data are available as Supporting Information). The strictly first order behavior observed for all transformations at all pH values forces us to conclude that only mononuclear species are involved. It is straightforward to propose that the slow step correspond to dissociation of a water ligand, leading to the cationic complex Cp*MoO2 + , see Scheme 1, [START_REF]follows: all k+ constants correspond to addition of oxygen species (H2O or OH -), all k-to loss of oxygen species; the subscript 1 corresponds to gain/loss of one proton, the subscript 2 to gain/loss of two protons[END_REF] which would therefore correspond to the dominant species at pH < 2. We shall now present further evidence confirming the presence of this cationic species before returning to the kinetic scheme.

The implication of the above proposition and of Scheme 1 is that the dinuclear complex Cp*2Mo2O5 ionizes in the 20% MeOH-H2O solution to initially produce a 1:1 mixture of the Cp*MoO2 + and Cp*MoO3 -ions , see Equation 1. Indeed, the spectrum of a Cp*2Mo2O5 solution in Figure 1 (trace a) is quite close to a linear combination of the Cp*MoO2 + spectrum (trace b) and that of Cp*MoO3 -(trace c). 33 The ionic nature of Cp*2Mo2O5 in 20% MeOH-H2O is confirmed by electrical conductivity studies, as shown in Figure 6. Under the validity of Equation 1, we wondered whether the known Cp*MoO2Cl complex would also ionize in the same medium (see Equation 2). The conductivity study of solutions of this compound fully confirm this hypothesis (Figure 6). M, as expected for strong electrolytes. However, they are very large and the slopes of these lines are greater than expected for the low concentration range explored in our case (typical salts have an almost negligible variation in this range). Moreover, the linearity is lost for lower concentrations, and the molar conductivities at infinite dilutions are very large (> 300x10 -4 Ohm -1 m 2 mol -1 ). Other 1:1 organic salts show conductivities in the range (80-115)x10 -4 Ohm -1 m 2 mol -1 in pure methanol for 10 -3 M solutions. [START_REF] Geary | [END_REF] Infinite dilution values are rarely above 150 for 1:1 salts except when hydronium ions are involved. All these features will in fact turn out to be completely consistent with our model. However, their full rationalization requires that we first detail the speciation of Cp*Mo VI in the entire pH range (section g).

Compound Cp*2Mo2O5 is soluble in all common organic solvents and essentially insoluble in pure water. This indicates that the left hand side equilibrium of Equation 1(dissolution of molecular Cp*2Mo2O5) is highly sensitive to the hydrophobic nature of the molecule. However, the right hand side equilibrium of Equation 1 is highly sensitive to the dielectric constant. This view is confirmed by the comparison of the molar conductivities in different solvents at a 4 . 10 -4 M concentration, see Table 1. Thus, the water-rich solvent mixture used in this study favors an essentially complete ionization.

(f) The complete scheme

For convenience, the formulas of complexes Cp*MoO3 -, Cp*MoO2(OH) and Cp*MoO2 + will be heretofore abbreviated as MoO -, MoOH and Mo + , respectively. The kinetic results described in sections (a) and (b) can now be reinterpreted according to Scheme 1, ultimately allowing us to understand and rationalize the kinetic and thermodynamic behavior of the Cp*Mo system in the entire pH range. The vertical transformations on the right hand side of the scheme are simple protonation/deprotonation equilibria and are rapid (probably diffusion-limited) on the time scale of the stopped-flow experiments. They can therefore be considered as a single species (abbreviated as MO, see Equation 3) for kinetic purposes. The kinetic evolution of the system under any pH condition can therefore be treated as a reversible first order equilibration between complex Mo + on one side and the species MO (the intimate nature of which is pH dependent) on the other side. The development of the kinetic scheme affords Equation 4for the overall rate constant of loss of oxygen-containing species (water and OH -) and Equation 5for the overall rate of the uptake of oxygen-containing species. The observed rate constant (kobs) is then simply given as the sum of k+obs and k-obs. The k+obs/k-obs ratio, on the other hand, gives an operational equilibrium constant Keq which will be pH dependent. This constant corresponds to the ratio between the sum of all oxygenated species (MO) and the nonoxygenated species Mo + .

Equation 3

[MO] = [MoO -] + [MoOH] + [MoOH2 + ] Equation 4 k +obs = k +1 K s [H + ] + k +2 Equation 5 k -obs = k -2 [H + ] k -1 + [H + ] + 1 + K 2 K 1 K 1 [H + ]
On the basis of this scheme, it is now possible to see that the acidification of MoO - /MoOH leads to Mo + by two different pathways, namely loss of OH -from MoOH and loss of water from MoOH2 + , the latter predominating at low pH. The pK2 (3.65±0.02) is available from the studies described in section (b), thus the term K2/[H + ] in the denominator of Equation 5 is negligible at pH < 3. The protonation of MoOH does not occur to a significant extent down to pH 0. This is clearly shown by the study in Figure 2, provided that species MoOH2 + would not have a spectrum perfectly identical to that of MoOH. This means that pK1 << 0 and the term [H + ]/K1 in the denominator of Equation 5is also negligible. Under these conditions, Equation 5 simplifies to the rate law k-obs = k-1 The reverse process (conversion of Mo + to MoOH/MoO -) takes place by the microscopic reverse of the two pathways mentioned above, namely nucleophilic addition of either H2O (k+2) or OH -(k+1) to Mo + . The expression of Equation 4shows that the latter pathway will prevail at high pH giving rise to a first order dependence on [OH -], as experimentally observed. Fitting Equation 4 to the experimental data gives k+1 = (3.260.04)x10 3 s -1 L. In order to elucidate the complete scheme, two rate constants are still missing: k-1 and k+2. However, these are linked to each other by the thermodynamic cycle of Scheme 1, as shown by Equation 6. The operational equilibrium constant, Keq, can therefore be expressed as in the simplified Equation 7, since pH > pK1 in all pH range (vide supra).

+ (k-2/K1)[H + ], or k-obs = (k-2/K1)[H + ]
Equation 6

k +1 k -1 = k +2 k -2 K 1 K s Equation 7 = k +2 k -2 K s [H + ] + [H + ] K 2 = k +1 K 1 [H + ] + [H + ] K 2 k -1 1 1 K eq
In principle, the missing parameter may be obtained in two independent ways. The first one is via the rate of equilibration in the intermediate region and subsequent fitting of kobs to the sum of Equation 4and Equation 5. The second one is via the measurement of the pH-dependent equilibrium [MO]/[Mo + ] and fitting of Equation 7. The first method cannot be applied since the equilibration rates are affected by a general acid/base catalysis in the buffered pH region (vide infra). The second method is described in the next section.

(g) The equilibrium in the intermediate pH range

The kinetic analyses in the extreme pH regions (< 2 and > 6) have provided us with the individual spectra of the pure species Mo + , MoOH and MoO -, see above. We are therefore in a position to deconvolute the equilibrium spectrum obtained for the kinetic runs at each pH into the sum of the spectra of the above individual species. This operation is conveniently carried out with the program SPECFIT [START_REF] Binstead | Specfit/32[END_REF] and is facilitated by the independent knowledge of the [MoO -]/[MoOH] ratio from the pK2 value. An excellent fit to the equilibrium spectrum was obtained at each pH value between 3 and 5. Outside this range, the equilibrium is too shifted to either side, e.i. Mo + or the MoOH/MoO -mixture (MO), to obtain any meaningful information. The experimental values of the MO/[Mo + ] ratio are shown in Figure 2 together with the best fit given by Equation 7. It is to be noted that the Keq curve has a transition from slope 1 to slope 2 in correspondence to the pK2 value. The fit of Equation 7provides the individual values of k-1 = (6.320.03)x10 -7 s -1 and k+2 = (5.010.12)x10 -4 s -1 . At this point, feeding all known rate and acidity constants into Equation 4 and Equation 5(the K2/[H + ] term in the denominator of this equation is neglected) provides the calculated curves shown in Figure 2.

From all derived rate and acidity constants (summarized in Table 2), it is now possible to obtain the detailed speciation of the Cp*Mo VI system at all pH values. The relative proportion of MO and Mo + is given by Equation 7and the relative proportion of species MoOH and MoO -is provided by the pK2 expression. Species MoOH2 + can be neglected. The result is graphically shown in Figure 7. According to this result, Mo + is essentially the only species in solution at pH < 2.5 while the MoO -is the dominant species at pH > 6. All species, including MoOH, are simultaneously present in the intermediate pH range. The maximum relative concentration of MoOH (ca. 15%) will be present at pH  4.

<insert Figure 7> It now becomes evident that, upon dissolution in 20% MeOH-H2O, Cp*2Mo2O5 will provide equivalent amounts of Mo + and MoO -and a small amount of MoOH, while the pH will self-regulate around 4.0 by hydrolysis of part of the Mo + , provided the solutions are sufficiently concentrated (> 10 -3 M). For more diluted solutions, a greater proportion of Mo + hydrolyzes to afford MoO -and twice the same amount of protons. A similar behavior occurs when the Mo + species is generated by the ionization of Cp*MoO2Cl. The concentrations of the different species H + , OH -, Mo + , MoOH and MoO -can be calculated in a straightforward fashion from all known equilibrium constants. The results are presented in Figure 8 a, b andc.

<insert Figure 8> At this point, we can analyze again and correctly interpret the results of the electrical conductivity study shown in Figure 6. In order to fit the conductivity data, the molar conductivities,  or  of every ionic species at each concentration were considered identical to the value    or    at infinite dilution. This is a reasonable hypothesis given our small concentrations (<8x10 -4 M). In addition, the values of   and   Cl-were fixed to the literature values (x10 -4 and x10 -4 S.mol -3 .m 3 , respectively) [START_REF] Janz | Nonaqueous Electrolyte Handbook[END_REF] as all attempts to keep them as free parameters led to instability. A global fitting of both Cp*2Mo2O5 and Cp*MoO2Cl curves with common molar conductivities yielded a very good agreement between the experimental and calculated curves according to our kinetic and thermodynamic scheme (see Figure 9), the values of   and    provided by the fit being (72±6)x10 -4 and (39±10)x10 -4 S.mol -3 .m 3 , respectively. These two values are in a reasonable range and are relatively close to each other, as one might expect from the similar size of the two ions. The determination of these values with a higher precision is prevented by the limited solubility of the compound and by the small contribution of these ions to the electrical conductivity at low concentrations in the presence of a significant amount of the more mobile protons.

<insert Figure 9> (h) The equilibration rate at intermediate pH: general acid/base catalysis.

In order to provide an independent verification of the numerical values for all rate constants, kinetic analyses were also attempted in the intermediate pH region. This required the use of a variety of different buffer solutions, as detailed in the experimental section. Given the results outlined in the previous section, we were obviously expecting to obtain overall equilibration rate constants kobs along the theoretical curve shown in Figure 2. As already mentioned, the transformations were found to follow perfect first order kinetics under all conditions, including the equilibrium region (2 < pH < 6). However, the observed values of the first order rate constants turned out infallibly greater than expected. This phenomenon may be attributed to a general acid/base catalysis operated by the buffer's non-dissociated acid(s) and/or its(their) conjugate base(s). Hydrogen bonding interactions between an undissociated acid and MoOH and MoOH2 + make the ligands OH - and H2O, respectively, better living groups, while the same kind of interactions between the buffer conjugate base and free water molecules or OH -ions make these better nucleophiles. This state of affairs is unambiguously proven by detailed studies at different buffer concentrations while keeping the pH and metal concentration constant. Examples are shown in Figure 10. At each given pH value, the extrapolation of the rate constants to a zero buffer concentration should afford a value in agreement with our developed scheme. This expectation is indeed verified for the experiments at pH 7.97 (TRIS) and 9.24 (borax), see Figure 2. In other cases, on the other hand, the extrapolation to zero buffer concentration is not sufficiently precise, or the extrapolated value remains significantly higher than the value which is predicted by our scheme. The latter phenomenon occurs in particular within the pH range 2-6. We speculate that this may be caused by the presence of significant equilibrium amounts of compound Cp*MoO2(OH), acting itself as a general acid/base catalyst. <insert Figure 10> (i) Nature of the MoOH2 + and Mo + species Two different tautomeric forms can be conceived for the Cp*MoO3H2 + ion, namely an aquo-dioxo and a dihydroxo-oxo form, see I and II. Although the water elimination process requires transit through form II, the most stable species may be either one of these two forms. The precise proton distribution for many inorganic aqua ions of molybdenum does not seem to be established with certainty. [START_REF] Richens | Group 6 Elements: Chromium, Molybdenum and Tungsten[END_REF] For instance, compound "MoO3" is often written as either Mo(OH)6 or MoO2(OH)2(H2O)2. However, if we assume that all proton transfer processes are extremely fast, then the possible involvement of an equilibration between the two tautomers would not affect the rate law of the overall transformation, no matter which of the two is the more stable species.

<insert I and II>

The nature of the Mo + species also warrants a brief discussion. First, from a kinetic point of view, it would not be possible to distinguish between Cp*MoO2 + and an adduct containing a solvent molecule. This molecule, however, could not be H2O since this is would be obtained directly by protonation of Cp*MoO2(OH) which, as argued earlier, should be an essentially diffusion-limited process. We should then consider the possibility that a MeOH molecule is coordinated to the Mo center. This possibility, however, is to be considered unlikely for the following reasons. This cationic species is present at a non negligible percentage in solution, in equilibrium with Cp*MoO2(OH) and Cp*MoO3 -, up to ca. pH 5 (see Figure 7). Since the pK of Cp*MoO2(OH2) + (pK1) was estimated as < 0 from kinetic evidence and since MeOH is only slightly less acidic than H2O (pKs = 15.5), the pK of an hypothetical Cp*MoO2(MeOH) + species could not be much greater than 0. This is incompatible with the existence of this species up to pH 5. For these reasons, we believe that the low pH species is in fact a bare Cp*MoO2 + cation. It is to be remarked that when both terminal oxo ligands participate in Mo-O bonding with both their available  lone pairs (thereby giving rise to a formal +2 triple bond), the Cp*MoO2 + complex achieves a saturated, 18-electron configuration. The loss of bonding to the H2O molecule would therefore be compensated at least partially by an increase of O-to-Mo  donation. Efforts to isolate a stable salt of this cationic complex and study its molecular and electronic structure are currently underway in our laboratory.

It is possible to find a parallel between the aqueous chemistry of Cp*Mo VI and that of inorganic molybdenum, when we notice that the (Cp* -)Mo unit is isoelectronic with the Mo(H2O)3 or with the Mo(H2O)2(OH -) unit. Thus, compound Cp*2Mo2O5 has a parallel in complex [Mo2O4(µ-O)(H2O)6] 2+ , while compound Cp*MoO2(OH) has parallels in complexes MoO2(OH)(H2O)3 + and MoO2(OH)2(H2O)2. [START_REF] Richens | Group 6 Elements: Chromium, Molybdenum and Tungsten[END_REF] The "unsaturated" complex Cp*MoO2 + would be related to a species such as MoO2(OH)(H2O)2 + or MoO2(H2O)3 2+ . Like the proton distribution discussed above, the precise hydration level is often unclear for molybdenum aqua ions. [START_REF] Richens | Group 6 Elements: Chromium, Molybdenum and Tungsten[END_REF] While aqueous "MoO3" is believed to adopt 6-coordination as mentioned above, its conjugate base is usually written as HMoO4 -(this obviously referring to a 4-coordinate MoO3(OH) -complex) and the corresponding dianion MoO4 2-is believed to be 4-coordinate. Thus, the unsaturation of Cp*MoO2 + should not be considered surprising.

Conclusions

The present investigation has uncovered the speciation of the Cp*Mo VI unit in the entire pH range in an essentially pure aqueous environment. The presence of 20% methanol is necessary to avoid the precipitation of neutral Cp*2Mo2O5 but does not directly participate in the speciation equilibria. The study has revealed the existence and stability as a function of pH of the hitherto unreported complexes Cp*MoO2(OH) and Cp*MoO2 + .

In addition, the involvement of a complex having formula Cp*MoO3H2 + as an intermediate in the generation of Cp*MoO2 + at low pH has been evidenced by the kinetic studies.

A parallel has been established between the aqueous behavior of Cp*Mo VI and inorganic Mo VI . An important difference, however, is related to the inertness of the Cp*-Mo bond, which resists hydrolysis down to pH 0. This has the consequence of blocking three coordination positions, rendering the Cp*Mo VI species unable to aggregate beyond the dinuclear level. Aqueous inorganic Mo VI , on the other hand, forms a large variety of polyoxomolybdate ions in the pH range 2-5. [START_REF] Richens | Group 6 Elements: Chromium, Molybdenum and Tungsten[END_REF] As a result, the speciation of Cp*Mo VI is relatively simple. The results presented in this contribution assist us in the interpretation of the reductive chemistry and electrochemistry of Cp*Mo VI in an aqueous environment, which is presented in another contribution. [START_REF] Gun | [END_REF]

Captions for Figures

  with a YSI model 35 conductimeter and a Beckman 35 Conductivity Cell. Stopped-flow kinetic measurements. The stopped flow kinetic runs were carried out with a Hitech SF-61-DX2 apparatus coupled with a Hitech diode-array UV-visible spectrophotometer. Kinetic traces were recorded at 25 °C in the pH range 1-13.25. The cell path length is 1 cm. Two types of manipulations were carried out as follows. Type 1 experiments (from Cp*2Mo2O5). A 2 . 10 -3 M Cp*2Mo2O5 stock solution was prepared in MeOH, since this compound is insoluble in water. For each stopped-flow kinetics run, the Cp*2Mo2O5 solution was prepared by diluting this stock solution with 4 volumes of H2O to obtain a 20:80 (v/v) solvent mixture. This operation was carried out no more than 1 min prior to injection into the stopped-flow cell. When the solution was allowed to stand for a long time (more than a half a day) at room temperature, the yellow starting compound reprecipitated. The above solution was mixed in the stopped-flow cell with an equivalent volume of a 20:80 (v/v) MeOH-H2O solution containing NaOH (for the kinetic run at pH ≥ 11.42) or a buffer at appropriate concentrations (see Results section) for the kinetic runs in the 7-10 pH region. Type 2 experiments (from Cp*MoO3 -). A 8 . 10 -4 M stock solution of Na + Cp*MoO3 -in 20:80 (v/v) MeOH-H2O was prepared from the MeOH stock solution of Cp*2Mo2O5 by dilution with 4 volumes of a diluted (ca. 10 -3 M) aqueous NaOH solution. This solution was mixed in the stopped-flow cell with an equivalent volume of a 20:80 (v/v) MeOH-H2O solution containing HNO3 (for the kinetic runs at pH ≤ 2.30) or a buffer at appropriate concentrations (see Results section) for the kinetic runs in the 2.5-5 pH region. Data analysis. All kinetic runs were analyzed with the program Specfit 28 while the fitting of the kinetic model to the observed first order rate constants and equilibrium constants as a function of pH (see Results) was carried out with the Excel Solver. The standard deviations on the determined parameters were obtained by means of the Solver Aid macro. 29 Results and Discussion (a) Kinetic study of the anation reaction of Cp*2Mo2O5. Solutions of Cp*2Mo2O5 in 20% MeOH-H2O are yellow and display an absorption spectrum which is characterized by a strong band in the UV with two shoulders at ca. 390 and 440 nm as shown by the trace a of Figure 1. The observed pH for this solution is ca. 4.

  . The slope of the log k-obs vs. pH is -1, namely the reaction is first order with respect to [H + ]. On the basis of this observation, it is relative straightforward to propose that Cp*MoO2(OH) undergoes a rapid and reversible protonation equilibrium to afford a Cp*MoO3H2 + intermediate, which then decomposes in a slow rate determining step. (d) Kinetics in the equilibrium range. The above studies have shown that a quantitative conversion to Cp*MoO3 -occurs at pH > 7 while a quantitative conversion to a new species occurs at pH  2. 30 Adjusting the pH at intermediate values, starting from either a Cp*MoO3 -or a Cp*2Mo2O5 solution, led to the evolution to a different and pH-dependent equilibrium situation. An example at pH 4.06 is shown in Figure 5. Trace a is the first recorded spectrum starting from the Cp*MoO3 -solution and corresponds therefore to the acid dissociation equilibrium mixture of Cp*MoO3 -and Cp*MoO2(OH). This solution evolves toward the equilibrium solution (trace c) following first order kinetics. Trace b is the first recorded spectrum starting from the Cp*2Mo2O5 solution (the true nature of which will be revealed in the next paragraph).

  (e) Nature of the low pH species and nature of Cp*2Mo2O5 in 20% MeOH-H2O.As seen above, the acidification of Cp*MoO3 -immediately affords Cp*MoO2(OH) which decays (quantitatively at pH < 3) to a new species with a rate law showing a first order dependence on the proton concentration. This behavior is interpreted as resulting from a fast and reversible protonation of Cp*MoO2(OH) to afford a Cp*MoO3H2 + intermediate, which then slowly evolves to a final product, now known to be mononuclear.

1

 1 Cp*2Mo2O5(s) Cp*2Mo2O5(solv) Cp*MoO2 + (solv) + Cp*MoO3 -(solv) + (solv) + Cl -(solv) Although qualitatively quite convincing, the conductivity data present some unusual features from the quantitative point of view. The molar conductivity values vary more or less linearly with [Cp*2Mo2O5] 1/2 or [Cp*MoO2Cl] 1/2 for concentrations greater than 4x10 -5

Figure 1 .

 1 Figure 1. UV-visible spectra of 20% MeOH-H2O solutions of (a) pure Cp*2Mo2O5 (pH

Figure 2 .

 2 Figure 2. Plot of the experimental log kobs (triangles) and log Keq (squares) vs. pH. The

Figure 3 .

 3 Figure 3. Spectra in 20% MeOH-H2O of solution obtained by acidification of Cp*MoO3 -

Figure 4 .

 4 Figure 4. Key UV-visible spectra for the Cp*MoO2(OH) evolution at pH 1 in 20%

Figure 5 .

 5 Figure 5. Key UV-visible spectra for the evolution of Cp*MoO3 -and Cp*2Mo2O5 at pH

Figure 6 .

 6 Figure 6. Molar conductivity of a 20% MeOH-H2O solution of Cp*2Mo2O5 (diamonds)

Figure 7 .

 7 Figure 7. Relative proportion of species Mo + , MoOH and MoO -at all pH values in 20%

Figure 8 .

 8 Figure 8. (a) and (b): Calculated concentrations of H + (line only), Mo + (circles), MoO -

Figure 9 .

 9 Figure 9. Measured and calculated conductivity in a 20:80 MeOH-H2O solution as

Figure 10 .

 10 Figure 10. First order equilibration constants kobs as a function of buffer concentration at

Figure 1 0

 1 Figure 1

  when loss of water from species MoOH2 + predominates,

i.e. at low pH. Indeed, this is the observed trend. Since pK1 < 0, individual values of k-2 and K1 cannot be obtained. Fitting the simplified rate law to the experimental data gives k-2/K1 = 17.260.22 s -1 .

Table 1 .

 1 Molar conductivies for 4 . 10 -4 M solutions of Cp*2Mo2O5 in different solvents. a

	Solvent	/Ohm -1 m 2 mol -1
	MeCN		0 b
	CH2Cl2		0
	EtOH		0.4
	Acetone		0.8
	MeOH		4.2
	20% MeOH-H2O	102
	Table 2.	Summary of all rate and equilibrium constants (Scheme 1).
	pK1	< 0
	pK2	3.65±0.02
	k+1	(3.260.04)x10 3 M -1 s -1
	k-1	(6.320.03)x10 -7 s -1
	k+2	(5.010.12)x10 -4 s -1
	k-2/K1	17.260.22 s -1

a Corrected for the molar conductivity of the pure solvent. b In this solvent, the measured conductivity was smaller for the solution than for the pure solvent.

Acknowledgement. We are grateful to the Ministère de la Recherche and to the CNRS for general support through UMR 5632, and to the Conseil Régional de Bourgogne for providing part of the funds for the purchase of the stopped-flow apparatus.


Funda Demirhan

Jenny Gun

Ovadia Lev

Alexandre Modestov

Rinaldo Poli
email: poli@u-bourgogne.fr

Philippe Richard

High oxidation state aqueous organometallics. Formation and structure of an oxocentred Cp*Mo V trinuclear cation by chemical reduction of Cp* 2 Mo 2 O 5

By zinc/CF3COOH reduction in MeOH-H2O, compound

High oxidation state organometallic chemistry is now a mature field with important applications to a variety of catalytic transformations. 1,2 Although many compounds in this class are stable in an aqueous environment and some have even been synthesised in water, their aqueous chemistry has rarely been investigated. Complex [Cp*2Mo2O5] 3, 4 is one such example. We have recently described an improved aqueous synthesis, 5 the speciation of this compound in water 6 which highlighted the new species [Cp*MoO2(OH)] and [Cp*MoO2] + , and an exploratory aqueous reductive electrochemistry study. An on-line voltammetric-ESMS investigation has indicated the formation of a wealth of new mixed oxo-Cp* complexes in oxidation states down to IV. 7 Therefore, we are now proceeding to explore synthetic chemical reductions under a variety of conditions. The first product that we have isolated, showing unexpected and unusual features, is reported in this contribution.

The compound is generated by zinc reduction in MeOH-H2O containing trifluoroacetic acid and air-stable single crystals have been obtained from THF-Et2O. † The structure of the compound has been elucidated by X-ray crystallography.

‡ It is composed of [(Cp*Mo)3(3-O)(2-O)4(2-O2CCF3)3] + (Fig 1)
and [Zn2(2-O2CCF3)4(O2CCF3)2] 2-(Fig. 2) ions. The cation, of ideal C3v symmetry, sits on a general position, while the dianion lies on inversion centers. The cation features an oxo-capped equilateral Mo3 triangle where each metal has the formal oxidation V (3 cluster electrons). Each edge is further bridged by an oxo and a trifluoroacetate ligand, and an  5 -Cp* ligand completes each metal's coordination sphere. No previous example seems available of a Mo3 cluster, whatever the coordination sphere and geometry, where the metal oxidation state is as high as V.

The central Mo3O4 core is a quite typical archetype for Mo3 clusters, but is mostly found for clusters containing 6 metal electrons (Mo IV ), e.g. the [Mo3O4(H2O)9] 4+ aqua ion. 8 Examples have also been described for lower oxidation states, notably 8electrons (Mo3 III,III,IV ), but not for higher ones. [START_REF] Richens | The Chemistry of Aqua Ions[END_REF] Previously reported clusters featuring triangular (Cp*Mo)3 units include [(Cp*Mo)3(2-OH)n(2-O)6-n] 2+ (n = 4,5,6; e.g. 5, 6 and 7electron clusters), [START_REF] Bottomley | [END_REF] the two related [Cp*4Mo5O11] and [Cp*6Mo8O16] clusters [START_REF] Bottomley | [END_REF]11 containing the same 4-electron Mo3(2-O)6 core, and [(CpMo)3(3-O)(2-Cl)4Cl3]. 12 None of these compounds contains a Mo3O4 core, nor has any of them the oxidation state V for all Mo centres. The related 14 are also known. The Mo3(2-O2CR)3 unit has precedents in non organometallic complexes with 6-9 cluster electrons, none of which contains a Mo3(3-O)(2-O)4 core. [15][16][17][18] The closest relative appears to be [Mo3O4(O2CH)8] 4-(another compound of Mo IV ), having a single bridging and seven terminal ( 1 ) formato ligands. 19 The Mo-Mo distance indicates little or no direct metalmetal bonding. The absence of a strong interaction may be easily understood on a qualitative basis, imagining the formal assembly of the [(CpMo)3(3-O)(2-O)3L6] cluster (L = terminal ligand) from three mononuclear CpML5 fragments (see Scheme 1). The Cp* ligand occupies the position trans to the capping O atom for each metal. Only one orbital (dxy) is available to accommodate metal electrons and the three resulting cluster 2)

Mo IV clusters [Cp3Mo3(3-S)(2-S)4] +13 and [(Cp*Mo)3(3-CH)(2-CH2)(2- O)2]
Mo(3) O(3) O(2) O (4)
O( 9) O( 10) O( 6) O( 5) O( 7) O( 8) CNT( 1) CNT( 2) The [Zn2(O2CCF3)6] 2-structure is also worthy of brief comment. Structurally characterised lantern-type Zn2(-O2CR)4L2 precedents (R = CH3, CH=CHCH3; L = py, py-4-CN, quin) show slightly shorter Zn-Zn distances (2.89-2.98 Å), [21][22][23] though a direct bonding interaction is precluded by the closedshell configuration of Zn 2+ . An incipient interaction between the Zn 2+ centres and the "free" C=O groups may be responsible for a slight misalignment between the Zn-O(term.) and Zn-Zn axes.

CNT(3)
Given the low coordinating ability of CF3COO -, its facile dissociation in an aqueous environment, leaving a [(Cp*Mo)3O4] 4+ core, may be expected.

However, an electrospray mass spectrometric (ESMS) study of the compound (positive mode) shows the envelope of the structurally characterised cation in either THF, MeCN, or MeCN-MeOH (see Fig. 3a). § MS n investigations reveal the successive loss of Cp*MoO2(O2CCF3), CF3COOH, CF3COOH, and H2O. Under negative detection mode, the major envelope corresponds to a [(Cp*Mo)3(O)4(O2CCF3)5] -species which derives from the tranfer of two CF3COO -ions from the Zn dianion. When the same experiment was carried out in the presence of 0.28 M CF3COOH, however, the monoanions [Zn(O2CCF3)3] -and [Zn2(O2CCF3)5] -were observed (see Fig. 3b).

The Cp*Mo V moiety in combination with oxo ligands is already known in dinuclear (e.g. [Cp*MoO2]2) 24 and mononuclear (e.g. [Cp*MoOCl2]) 25 complexes. Dinuclear and mononuclear oxo-carboxylato Cp*Mo V species were also detected in our recent EC-ESMS study, 7 whereas a trinuclear one was not. Whether a relationship between these smaller fragments and the trinuclear complex shown here exists and under which conditions one may be able to interconvert these species are questions for future investigations. Another interesting and related question is the lability of the capping oxo ligand, since this is extremely inert in the large class of Mo IV 3O4 clusters. 8 We also wish to isolate larger quantities of salts containing this cation to investigate its magnetic properties. As a final remark, we underline the robustness of the Cp*Mo bond, which remains intact upon long reaction times under strongly acidic aqueous conditions. This feature is encouraging for the future development of catalytic and electrocatalytic processes with Cp*Mo systems in an aqueous environment. JG and RP thank Arc-en-Ciel/Keshet and FD thanks TUBITAK-NATO for travel grants.

Notes and references † [Cp*2Mo2O5] (0.15 g, 0.28 mmol) dissolved in MeOH:H2O (1:1, 10 ml) was treated under argon with Zn powder (1.5 g, 23 mmol) and concentrated CF3COOH (10 drops). Three-day stirring at rt yielded a green suspension. After filtration and drying, the solid was extracted with THF and, after filtration, addition of heptane yielded a blue precipitate. A small amount of single crystals of the title compound were obtained by diffusion of an ether layer to a blue THF solution at rt.

Fig. 2

 2 Fig. 2 ORTEP view of the [Zn2(O2CCF3)6] 2-anion. Relevant averaged parameters: Zn•••Zn, 3.228(5), Zn-O(bridging), 2.05(2); Zn-O(terminal), 1.926(3); C-O(br.), 1.214(14); C-O(term.), 1.243(5); C=O, 1.207(5) Å; O(br.)-Zn-O(br.)' (cis), 86.4(9); O(br.)-Zn-O(br.)' (trans), 150.9(2), O(11)-Zn-Zn#, 166.7(1) °.

Fig. 1

 1 Fig. 1 ORTEP view of the [Cp*3Mo3O4(O2CCF3)3] + cation. Relevant averaged parameters: Mo-Mo, 3.1287(15); Mo-CNT (Cp* centroid), 2.069(5); Mo-O(1), 2.032(6); Mo-(2-O), 1.95(1); Mo-O(acetate), 2.183(10); C-O, 1.249(4) Å; CNT-Mo-O(1), 177.63(12); Mo-(2-O)-Mo', 106.9(4); Mo-(3-O)-Mo', 100.7(3)°.

Scheme 1 .Fig. 3 .

 13 Scheme 1. Orbital interactions in [(CpMo)3(3-O)(2-O)3L6] clusters

  MS investigations were carried out with a Finnigan (San Jose, USA) LCQ quadrupole ion trap mass spectrometer equipped with a Protana (Odense, Denmark) nano-electrospray ionization (ESI) interface. Tip and sampling cone potentials were -1.5 kV and -50 V (negative mode), 1.5 kV and 23 V (positive mode), respectively. The mass analyzer was scanned from m/z 100 to 2000 (5 total microscans). Maximum injection time into ion trap was 50 ms. Sample injection flow rate was 1 L*min -1 .

	‡ Crystal data. C42H45F18O16Mo3Zn, M = 1500.97, monoclinic, a =
	11.8671(1), b = 11.7975(1), c = 37.6903(5) Å,  = 95.767(1)°, U =
	5250.01(9) Å 3 , T =110 K, space group P21/n (no.14), Z = 4, (Mo-K) =
	1.28 mm -1 , 20526 reflections measured, 11521 unique (Rint = 0.039)
	which were used in all calculations. The final wR(F 2 ) was 0.096 (all
	data).

§ H. Arzoumanian, A. Baldy, M. Pierrot, and M. Petrignani, J. Organometal. Chem., 1985, 294, 327. 25 F. Bottomley, P. D. Boyle, and J. H. Chen, Organometallics, 1994, 13, 370.

) (LINE INCLUDED FOR SPACING ONLY -DO NOT DELETE THIS TEXT) Cp*2Mo2O5 yields a salt which is composed of [Cp*3Mo3O4-(O2CCF3)3] + and [Zn2(O2CCF3)6] 2-ions after crystallisation from THF/Et2O. coordination environment and electronic structure preclude the establishment of strong metal-metal interactions.