Coordination of a polyfunctional cyclic ligand containing a PP bond to the fragment [Mo(CO)3(NN)] (NN = 2,2′-bipyridine, 1,10-phenanthroline). Spectroscopic and electrochemical characterization

Polyhedron Vol. 7, No. 6, pp. 489494, Printed in Great Britain

1988 0

0277-5387/88 %3.00+.00 1988 Pergamon Press plc

COORDINATION OF A POLYFUNCTIONAL CYCLIC LIGAND CONTAINING A P-P BOND TO THE FRAGMENT [Mo(CO),(NN)] (NN = 2,2’-BIPYRIDINE, l,lOPHENANTHROLINE). SPECTROSCOPIC AND ELECTROCHEMICAL CHARACTERIZATION J. GRANIFO*

and M. E. VARGAS*

Departamento de Ciencias Quimicas, Facultad de Ingenieria y Administration, Universidad de La Frontera, Casilla 54-D, Temuco, Chile J. COSTAMAGNA*

Departamento

de Quimica, Facultad de Ciencia, Universidad de Santiago de Chile, Santiago 2, Chile and M. A. FRANCOIS

Departamento

de Ciencias Naturales, Area de Q&mica, Pontificia Universidad Catolica, M.Montt 56, Temuco, Chile (Received 29 July 1987; accepted 13 October 1987)

Abstract-The reaction of 1,3,4,6-tetramethyl-lH,4H-1,3,4,6-tetraaza-3a, 6a-diphosphapentalen-2,5(3H,6H)-dithion-3a-sulphide (TDP) with the derivatives of metal carbonyls [Mo(CO),(NN)(CH,CN)] (NN = 2,2’-bipyridine, 1, lo-phenanthroline) leads to f’cMo(CO),(NN)(TDP) complexes with the ligand coordinated through the trivalent phosphorus atom. The IR and visible spectra of the new complexes as well as their electrochemical and general properties are discussed.

In a recent paper Kleeman et al. ’ have reported that the reaction product from N,N’-dimethylthiourea and phosphortrichloride is the compound tetraazadiphosphapentalene (TDP) containing a cyclic system involving a P-P bond. (See Structure 1.) As can be seen, TDP is a potential multiple ligand with possible donor positions at the N, S and tervalent P atoms. Hitherto, there are no works described in the literature dealing with the coordination form of this interesting polyfunctional ligand uniting the characteristics of thioureas, phosphines and sulphur phosphines. Mixed derivatives of the formula ,fac-[Mo(CO), (NN)L] [NN = 2,2’-bipyridine (bipy) or 1,l O-phen-

* Authors to whom correspondence

should be addressed.

anthroline (phen)], L being a wide range of monodentate Lewis bases are well known.2-g These trisubstituted complexes are usually obtained through thermal displacement of one carbonyl group by direct reaction of Mo(CO),(NN) with an excess of L in high boiling solvents.7 Nevertheless, in order to protect the sensitive ligand TDP from thermal decomposition, a less drastic procedure has

Structure

1.

490

J. GRANIFO

et nl.

Table 1. Analytical and IR spectra for the complexes [Mo(CO),(NN)(TDP)] Analysis (%)

v(CO)b

Compound

C

H

N

MO

A,

E

PWW4biw)VWl DWWdphen)UWl

35.8 (36.0) 38.3 (38.3)

3.2 (3.2) 3.1 (3.1)

12.9 (13.2) 12.6 (12.8)

15.2 (15.1) 14.3 (14.6)

1955 1930

1860, 1810 1840, 1805

a Required values are given in parentheses. bNujol mulls, in cm- ‘, all bands are strong.

been employed in the present work for obtaining tricarbonyl derivatives by substitution of acetonitrile in the complexes fuc-[Mo(CO),(NN) (CH$N)].’ RESULTS

AND

DISCUSSION

The reaction of [Mo(CO),(NN)(CH,CN)] with TDP in dichloromethane at room temperature, gave the new complexes of the formula [Mo(CO),(NN)(TDP)]. These red coloured crystalline solids are air stable and soluble in organic solvents such as benzene, methylene chloride and chloroform ; slightly soluble in alcohols and unsoluble in light petroleum. However, their solutions are unstable in air. IR spectra IR spectra of the complexes [Mo(CO),(NN) (TDP)] present two intense bands in the CO stretching region. The one of lower frequency is split into two (Table 1) producing a pattern similar to that from the complexes fuc[Mo(CO),(NN)L], L being a phosphorus donor ligand (specifically some phosphites).7 The behaviour of the vibrational modes A, and E of these complex fuc-tricarbonyls have been interpreted through considerations of the differences in the bond properties of the ligand groups NN and L.‘j

If the A-acceptor capacity of L is superior to that of NN, then the Emode is split. This effect increases with the rc-acidity of L and, simultaneously, the CO bands are shifted to higher frequencies. The results obtained for the complexes [Mo(CO),(NN)(TDP)] suggest the existence of a considerable difference in n-acceptor capacity between the substituent groups (TDP > NN). Furthermore, if the reported magnitudes for the phosphites7 are compared with those found for TDP, this shows a slightly greater rracidity. Accordingly, this conclusion permits us to discard the fact that TDP coordination occurs through their thiocarbonyl or thiophosphoryl groups, since they are weaker n-acceptors than phosphites or phosphines.7s’0 The other regions of the IR spectra show the presence of the bands assigned to TDP (v.g. : 1420(s), 1105(s) and 855(s) cm-‘), in some cases superimposed on those of the bipy or phen. Electronic spectra in the visible region (a) Band assignments. The visible spectra in solution of the complexes Mo(CO),(NN)(TDP) are dominated by a broad and intense band (E > 4000 M-’ cm-‘). Th e position of this absorption is strongly sensitive to the used solvent. The more polar the medium, the more the band is shifted to the blue (Table 2). Bands with similar characteristics have been assigned to metal to ligand charge-

Table 2. Visible absorption maxima (cm- ‘) of the complexes [Mo(CO),(NN)(TDP)] [Mo(CO),(NN)] in several solvents

Compound

PWC%(bipy)(TDP)l [MWWphen)(TW1

[MWCW-$v)l bWW.&hen)l

Benzene (0.34)

CHCl, (0.42)

20 000 20 040 20310 20 390

20 200 20 120 20 520 20 790

LISolvent parameters E;trLcr from ref. 11. ‘Complex too unstable in this solvent. ’ Data from ref. 11.

Solvent (E&_,& Butanol CH,Cl, (0.55) (0.67) 20 490 20 620 20 780 20 830

20 750 20750 21260 21390

and

Acetone (0.82)

CH,CN (0.98)

21370 -b 22010 22 170

21650 21600 22 520 22 830

Coordination

of a polyfunctional

transfer transitions (MLCT) NN(rr*) + Mo(4d) in the complexes [Mo(CO),(diimine)(PRJ]. ‘2,‘3 The solvatochromic behaviour of the related complexes Mo(CO),(diimine) (M = Cr, MO, W; diimine = bipy, phen or derivatives) has been employed by Manuta and Lees’ ’ in order to derive a scale of solvent polarity. They have defined the parameter EGLCTand correlated it with the absorption energies NN(n*) t M(d) of these complexes. The values of EcLCT are displayed in the interval given by the isooctane (E&= = 0.00) and DMSO of this solvent (E&r = 1.00). The application polarity scale to the complexes [Mo(CO), (NN)(TDP)] by plotting their absorption maxima &XT) vs E&CT leads to a linear relationship similar to that observed for the parent complexes Mo(CO),(NN) (Fig. 1 and Table 2) and given by the equation VMLCT= A + B * EcLCT. When the parameters of the corresponding straight lines of the complexes Mo(CO),(NN) (bipy : A = 18983, B = 3571 cm-‘, r = 0.991; phen : A = 19021, B = 3775 cm-‘, r = 0.982) and Mo(CO),(NN)(TDP) (bipy : A = 19062, B = 2668 cm- ‘, r = 0.993; phen: A = 19160, B = 2476 cm- I, r = 0.993) are compared, a good correlation is observed in all cases, evidencing that these transitions are of the same nature. On the other hand, the values of the slope indicate that the tetracarbonyl complexes are more sensitive to the solvent changes than the tricarbonyls. This fact can be explained by considering that solvatochromism occurs if the solute-solvent interactions in the ground state differ from that in the excited state of the substrate. In particular, the strongest absorption in the visible region of the complexes Mo(CO),(NN) (C,, symmetry) comes

491

cyclic ligand

r

do

/x

/GO ---

--f ,

/ , / + co

co

-2

Structure 2.

from the transition bz(rc*) t /I,(&z).‘~,‘~ (See Structure 2.) This MLCT transition induces a dipole moment in the excited state opposed to that of the ground state (z direction). Hence, a less polar situation occurs for the complexes in the excited state reducing the solute-solvent interactions. The expected stabilization of the ground state by polar solvents appears evident from the extensive blue shift of this transition in such media, i.e. negative solvatochromism.‘2*‘3 The substitution of one carbony1 group by a phosphorus-donor ligand destabilizes the metal orbitals ; consequently, the electronic transition from the metal to the ligands diminishes its CT character and hence its negative solvatochromism is less intense. The effect of the destabilization of the metal orbitals can be clearly appreciated in Fig. 1, where the wavenumbers of the tetracarbonyl absorptions are always higher than those of the corresponding tricarbonyls. (b) Ligand exchange reactions in acetonitrile. The dissolution of the complexes [Mo(CO),(NN) (TDP)] in acetonitrile produce slow colour changes

qMLCT (cm-’ 1 24000 . .

0.0

.2

I .4

1 .6

I .8

, 1.0

E;,,

Fig. 1. Wavenumbers of maxima MLCT absorptions (cMLcT) of [Mo(CO),(NN)] and [Mo(CO),(NN)(TDP)] complexes plotted against E tLCr values : + (---I PWCOMbipy)l, 0 (-) bWCOMphen)l, 0 6--) [Mo(COMbiw)(TDPI1 and 0 (--) [Mo(CO),(phen)(TDP)].

492

J. GRANIFO

ef al.

of the solution from yellow to violet. The position of the maximum in the visible region is shifted to lower energy with an isosbestic point in the intermediate region. The spectrum of the final solution is superposed on that of a sample of the complex [Mo(CO),(NN)(CH,CN)]. Therefore, it is concluded that there is competition between TDP and the solvent for the same coordination site. [Mo(C0)3(NN)(TDP)]

+ CH,CN

= [Mo(CO),(NN)(CH,CN)]

+ TDP.

(1)

The ligand exchange reactions in the metal carbony1 complexes of the 6B group are usual and the mechanisms of such reactions have been suggested. ’ 4 Under the experimental conditions used (CHJN as solvent), the reaction should have first-order or pseudo-first-order kinetics. The rate was measured by following the absorbance at 500 nm; the plots of In (A -A,) vs time lead to straight lines and the corresponding rate constants were extracted from the slopes (&,(bipy) = 5.4 x 1O-3 s- ’ ; k,,,(phen) = 1.8 x lo-* s- ‘) confirming the predictions of the mechanism proposed for these reactions. ’ 4 Electrochemistry The cyclic voltammograms of [Mo(CO),(NN) (TDP)] complexes in CH2C12 show an irreversible oxidation peak which depends on the scan rate ; a shift towards a more anodic direction is observed when the scan rate is increased (Fig. 2). The coulometry in the same solvent through an exhaustive oxidation (0.99 V vs SCE) results in the loss of 0.5 electrons per MO, giving a species with an electronic spectrum that resembles the initial

400

500

600

I.,.,.,

a.,.,.,.

1I’I-I’

II.1.I.I

0.0 0.5 1.b 1.5 0.0 0.5 1.0 1.5

E(V) vs SCE Fig. 2. Cyclic voltammograms of (a) [Mo(CO), (phen)(TDP)] and (b) [Mo(CO),(bipy)(TDP)] in dichloromethane. The values indicate the scan rates (mV s- ‘).

compound except for the presence of a broad and weak band at long wavelengths and slight displacement of the others (Fig. 3). It has been reported that the Mo(C0)4(bipy) complex in CHzC12 suffers an oxidation by a quasireversible process giving the cation [Mo(CO), (bipy)]+ (El,* = 0.62 V vs SCE).” However, we have observed that the complexes Mo(CO),(NN) in CH&N present the oxidation in an irreversible way (EJbipy) = 0.68 V, E,(phen) = 0.67 V at 200 mV s- ‘). This can be understood assuming that the molybdenum atom has the possibility to obtain heptacoordination by reacting the monocation with a molecule of the donor solvent to yield species of the formula [(Mo(CO),(NN)(CH,CN)]+. Similar behaviour in the same solvent has been observed in the related complexes [Mo(CO), (bidentate phos-

700

loo0

BOO h,

nm90°

Fig. 3. Electronic spectra of [Mo(CO),(phen)(TDP)] solutions before (---) and after (trolled-potential electrolysis in dichloromethane (0.99 V vs SCE).

) con-

Coordination the oxidized phine)]. I6 Furthermore, generated species derived from MOM also react with CH$N. i6*l7

of a polyfunctional electro-

may

Accordingly, these anticedents suggest that the cations produced from the molybdenum carbonyl derivatives have a great tendency to increase their coordination number if the medium proportionates some adequate donor species. Under this premise it is possible to make clear the electrochemical behaviour of the complexes [Mo(CO),(NN)(TDP)] in CH,Cl, through the following reaction mechanism : PWW

,WWTWI + [Mo(CO),(NN)(TDP)]+

+e

(2)

+ W(COh(W(TDP)I+ + [Mo(CO),(NN)(TDP) -

Mo(WdNN)(TWI

+.

493

cyclic ligand

added 190 mg (0.637 mmol) of TDP and was stirred

for 30 min. The dark red coloured solution was then filtered. To this filtrate 10 cm3 methanol were added, followed by vacuum evaporation to eliminate the major part of the dichloromethane. The red crystals were separated by filtration and washed with three portions of methanol (10 cm3) and dried in vacuum (yield: 327 mg, 78%). The elemental analysis of these compounds are given in Table 1. Electrochemical

measurements

All voltammograms were recorded in CH2C12 or CH3CN (0.1 M Bu,NClO,) on a Model 173/175 PAR electrochemistry system. A three-electrode cell consisting of a platinum-disc working electrode, a platinum wire counterelectrode and an Ag/AgCl (in Me,NCl, f0.01 vs SCE) reference electrode was used. Controlled-potential electrolysis experiments were performed with a PAR 179 coulometer.

(3)

The first step should be the electrochemical generation of the monocation, which in a second fast step, is coordinated by a molecule of the neutral complex through the ligand TDP. Therefore, this acts as a bridge by way of one of its sulphur atoms producing a binuclear species. Then, the net result of the oxidation must be the elimination of 0.5 electrons per MO. The electronic spectra of the oxidation products seem to confirm the above hypotheses (Fig. 3). If the final spectrum results from the superposition of both ; that of the oxidized species with that of the neutral one, this might explain the great similarity with the spectrum of the non-electrolyzed substance. EXPERIMENTAL The solvents were purified by standard methods and all operations were carried out under a nitrogen atmosphere. The complexes Mo(CO),(NN), Mo(CO),(NN)(CH,CN)~ and the ligand TDP’ were prepared by methods described in the literature. The JR spectra were obtained in a PerkinElmer 577 spectrophotometer. The electronic spectra were recorded on a Carl-Zeiss DMR 22 instrument.

Acknowledgements-Financial support given by the Direction de Investigation de la Universidad de La Frontera (Project N” 1032126), the Comision National de Investigacibn Cientifica y Tecnologica (Project N” 5007/85) and the Dire&on de Investigaciones Cientificas y Tecnologicas de la Universidad de Santiago to carry out this work is gratefully acknowledged. A gift of some metal hexacarbonyls from Dr H. Mtiller of Merck Quimica, Chile is also gratefully acknowledged.

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433.

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(NN = bipy,

An example of the general procedure is the following. Mo(CO),(phen)(TDP) : Into a solution of 225 mg (0.635 mmol) of just prepared Mo(CO), (phen)(CH$N) in 10 cm3 dichloromethane were

J. Chem. Sot., Dalton Trans. 1973,236O.

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