IR study of hydrogen bonds formed by π-complexes of transition metals in liquid xenon solution

JournalofMolecularStructure,174 (1988) 29-34 Elsevier SciencePublishersB.V.,Amsterdam-PPrintedinTheNetherlands

IR STUDY

0~ HYDROGEN

BONDS

29

FORMED SYJ~--COMPLEXES 0~ TRANSITION

METALS IN LIQUID XENON SOLUTION B.V.LOKSHIN, S.G.KAZARIAN, A.G.GINZBURG A.N.Nesmeyanov Institute of Organo-Element Compounds, Academy of Sciences, Vavilova Str. 28, 117813 Moscow (USSR) ABSTRACT By the me thod of IR spectroscopy the interactioc of various ff-comvlexes of transition metals with proton donors in liquid Xe solutibns at 165-270 K have been studied. Perfluoro-tert.-butanol (PFB) and HCl were used as proton donors, since they are not associated in liquid Xe. Ferrocene and its alkyl derivatives form Hbonds of OH... ff type with PFE with participation ofT-electrons of the cyclopentadienyl ring. For carbonyl and nitrosyl complexes the new types of H-bonding M-CO...HA and M-NO...HA with the oxygen atoms of carbonyl and nitrosyl ligards have been found. It is shown that H-bond with the oxygen atom might be the first step in the reaction of proton transfer to the metal atom. INTRODUCTION The application of the liquefied noble gases as solvents for spectroscopic measurements was started not long ago (ref. l).These solvents have been used for studying photochemical reactions of organometallic compounds

(ref.2,3) and the hydrogen bonded complexes

with orgar,ic molecules as proton acceptors (ref. 4). In this work the solutions in liquid Xe (LXe) have been used for the study of intermolecular interactiors ofr-complexes

of transition metals

with proton donors, taking into consideration the advantages of such a solvent (high tracsparency over a whole IR spectral region, low solvatation capability). EXPERIMENTAL For IR spectroscopic experiments in LXe the special high pressure (up to 50 atm) cryostat with BaF2 windows and 7.5 cm optical pathlength was constructed, permitting the measurements in the temperature range of 165-270 K. IR spectra were recorded on a Bruker IFS-113~ fourier transform spectrometer. As proton donors perfluoro-tert .-butanol (CF3J3COH and HCl were used since they do not form self-associates in LXe solutiors at low temperatures. Molar concentrations of compounds were 10-4-10-5.

0022.2860/88/$03.50 O1988ElsevierSciencePublishersB.V.

30

RESULTS AND DISCUSSION Hydrogen bonding of PFB bzithferrocene derivatives in liquid Xe The interaction of PFB with ferrocene and its alkyl derivatives have been studied. Appearance of the new band which is red-shifted -1 as compared Qth the free PFB $(OH) band (3590 cm ) and the increase of its integral intensity demonstrate

that the formation of

H-bonded complexes takes place (see Table 1). The entl-alpies of Hbonding have been determined from the temperature dependence of spectra. TABLE 1 OH stretching frequencies and shifts, equilibrium constants and enthalpies of H-complex formation of ferrocenes with (CF3)3COH.

3,cm-l

Compound

K eq,l/mole n3,cm-l

~H,kcal/mole 175 K

Ferrocene Dimethylferrocene Diethylferrocene Hexamethylferrocene Octamethylferrocene Decamethylferrocene

3455 3440 3440 3428 3418 3407

135 150 150 162 172 183

35 85 85 185 255 410

210 K 10 20 20 42 47 60

3.0 3 :6 3.6 3.8 4.3 4.8

Comparison with previous data for H-complexes of PFB k.itholefines and aromatic compounds permits to conclude that the most probable electron donors in H-bonding with ferrocene derivatives are the cyclopentadienyl rings. The H-bond belongs to the fl...HO type. H-bond of a new type MCO... HA in transition metal carbonyls It is well known, that transition metal carbonyl complexes can be protonated at the metal atom by protonic acids. It has been postulated, that the possible intermediates of these reactions are the complexes with H-bond at the metal atom as proton acceptor. In the present mrk

we have not found the formation of the M...HA in-

termolecular bonds. Our data demonstrate, that the protonation at the metal atom can pass through the stage of H-konding at the oxygen atom of the carbonyl ligand. Up to now H-lronding has not been known for the metal carbonyl group, possibly because of the low basicity of oxygen atom due to the considerable contritution of the resonance structure I to the structure of metal carbonyl group: M--GO+ (I)c----+M=C=O (II). The first example of H-bond of M-CO.. .HO type was found upon the

31

interaction of complex CpMn(C0)2P(i-Pr)3

(I) (Cp= $ZJ~-C~H~)with

PFB in LXe solution. At temperature decrease a new downshifted

(OH)

band appear which indicates the formation of the H-bond. Carbonyl complexes'of such a type have three different centers of basicity, potentially capable to H-bonding:$-ligand,

metal atom and oxygen

atom of carbonyl group. IR spectroscopy permits to distinguish these cases. The H-bond at the g-ring

or at the metal atom leads to

some increase of the positive charge of the metal atom and consequently to a high-frequency shift of ?(CO). But in our experiments new bands in the ?(CO) region of IR spectra appear, shifted to the low-frequency region (Fig.1). Study of the temperature dependence of the spectra and force constants calculations for CO bonds show -1 that the new bands 1835 and 1931 cm belong to I:1 H-complex,where the H-bond is formed by one of the CO groups. The bands 1832 and 1906 cm-1 have been assigned to 1:2 complex, where both CO groups are H-bonded (1832 cm -1 band is obscured by the more intense 1835 -1 cm. band of 1:l complex and can be separated by difference spectroscopy).Thus, in the present case the formation of an unknown up to now type of H-bond with the oxygen atom of CO group at the metal atom has been found. The enthalpy of H-bond for 1:l complex determined from temperature dependence of IR spectra is

6 kcal/mole.

Later we found that the formation of M-CO...HO bonds is typical for metal carbonyls of types CpM(CO)3,CpM(C0)2L,CpM(CO)L2

(L= phos-

A

2000 Fig.1

1950

1900

1850

1800 cm-l

?(CO) region of the LXe solution of IR spectra in the CpMn(C0) P(i-Pr) in presence of PFB: (a) T= 210 K; (b) T= 180 K; (6, T= 162 K.

32

phine, Cp=$-C5H5

or q5-C5Et5), AreneM(COJ3 and some other. Some

examples are presented in Table 2 for the complexes containing the group M(CO)3 with C3v local symmetry. TABLE 2 Frequencies of CO stretching vibrations for some metal carbonyls and their H-complexes with PFB and S(OH) shifts in LXe solutions.

Compound

Initial compo?C) C3v symmetry E Al

rcm-1 H-complex 1:l CS symmetry A'

A"

- bq(OH),cm-1 A'

;5;5g;::))3 55 3 C5Et5Mn(CO)3 C5Et5Re(C0)3

2031 2029

1942 1941

2029 2031

1952 1958

1906 1913

2010 2015

1930 1925

2010 2015

1940 1936

1892 1887

95 105

C6Me3H3Cr(C0)3 C6Me3H3Mo(CO)3

1972 1976

1903 1906

'1972 1976

1915 1913

1862 1855

120

’

The following changes in the spectra are typical: low frequency 3(CO) band is splitted, one of the components is 30-50 cm-' downshifted and the other one is 7-12 cm-1 upshifted as compared to the initial position.The model calculations of CO group force constants are in agreement with the MCO...HO H-bonding. Taking into consideration that the

$ (OH) frequency shift is parallel to the strength

of H-bond it may be concluded, that in the serie of the isostructuralN-complexes

the basicity of the oxygen atom of the carbonyl

ligand increases upon the introduction of the donor substituents into *-ligand

talky1 groups), the exchange of Mn atom to Re, the

substitution of the CO groups for more donor phosphine ligand (for complex I the 4 (OH) shift is 175 cm-'). For some of the*-complexes

mentioned it is possible to observe

H-bonding at the CO ligand with PFB not only in LXe but also at room temperature in common organic solvents, but the infofmativity of the IR spectra in these cases is considerably less than for LXe solution because of the broadening of the absorbtion bands. The formation of H-bands of MC0 ...HCl type in the LXe solution has been observed also at the interaction of fl-complexes with HCl. Spectral changes in the $ (CO) region in this case are the same as for reaction with PFB, but the frequency shifts are somewhat less in accordance to the lower acidity of HCl in comparison with PFB.

33 H-bonding and protonation of carbonyl complexes Diphosphine monocarbonyl complexes CpMn(CO)L2 , where L2=(PPh3)2, @Et31 *, Ph2P(CH2),PPhZ (n=l-31, are more basic than corresponding di- and tricarbonyl ones. For these compounds upon the interaction with PFB or HCl in LXe apart from H-bonding the protonation at the metal atom has also been observed. The protonated molecules appear in the solution after its freezing (T=163 K) and the consequent melting of LXe solution (when the measurement of the IR spectra becomes possible).The data are presented on Fig.2.At 230 K CpMn(CO)* (PEt3J2 (II) does not interact with PFB. At 165 K II practically completlytransforms to the H-bonded form. The $(CO) frequency shi-1 fts from 1840 to 1781 cm . After freezing and melting of the solu-1 tion at 165 K the new band at 1910 cm appear, corresponding to the protonated form. Since before freezing the molecules of II were completly H-bonded, the protonated complex is formed from the Hcomplex. While freezing the "cryoconcentration" takes place and the concentrations of the solutes increase sharply in the liquid phase. Under such conditions the molecules of PFB, beeng in excess,take off the RFO- anion from the H-complex and the rearrangement proceeds with proton transfer from the oxygen atom to the metal atom: MCO...BORF + R~OH b

(~o)~H-

OC-M+-H .

+ (MCOH+I +

Alternative variants of the proton transfer are possible, but the protonation is sure to proceed with the participation of the H-bonded complex at the oxygen atom of CO ligand. 7

a T b

-1 1750 cm Fig.2. IR spectra in the (CO) region of LXe solution of CpMn(C0) (PEt ) in the presence of PFB:(a) 230 K,(b) 180 K, (c) 165 K, (dJ2165 K after freezing and melting. 1950

1850

34

H-bond of MNO... HA type for carbonyl nitrosyl complexes Upon interaction of PFB and HCl withT-complexes

CpM(C0)2NO

(MC

Cr, MO) in LXe solutions the formation of the new type of H-bond MNO... HA with the oxygen atom of nitrosyl ligand have been found. -1 Spectroscopicaly such a bond is characterised by 30-40 cm low frequency shift of ?(NO), lo-15 cm-' high frequency shift of s(CO) and low frequency shift of

$(OH) of PFB or

$(HCl) of HCl. There-

fore the nitrosyl oxygen atom is more basic than the carbonyl ones. In the reaction of CpCr(C0)2NO with hC1 in LXe the protonation at the chromium atom also has been observed. It is accompanied by the strong high frequency shift 0100

cm-l) of either q(NO) or

s(CO)

bands. H-bonding with bridged metal carbonyls For the dimeric carbonyl compound

[CpFe(CO)2]2 (III) ,containing

two terminal carbonyls and two bridged ones, in the presence of PFB or HCl in LXe the consequent formation of two H-complexes with one or both bridged carbonyl groups has been found. The j(CO) frequencies of the terminal carbonyls increase from 1962, 2007 cm-l to 1973,2015 cm-' and 1983, 2022 cm-1 for H-complexes 1:l and 1:2,respectively. Bridged carbonyls in III show only one q(CO) band 1795 -1 (symmetric one is not active in the cm of asymmetric j(CO) mode IR spectrum in accordance with the local symmetry D2h of the central Fe(C0)2Fe unit). Under H-bonding this frequency shifts to 1733 cm-'for both 1:l and 1:2 H-complexes. The 1817 cm-'band of unbonded bridged carbonyls for 1:l H-complex also has been observed. The results are in accordance with the higher basicity of the bridged carbonyls as compared to the terminal ones. REFERENCES 1 2 3 4

M.O.Bulanin,J.Mol.Struct.,l9 (1973) 59. J.J.Turner,M.Poliakoff, Z.Anal.Chem.,324 (1986) 819. R.R.Andrea,H.Luyten, M.A.Vurman, D.J.Stufkens,A.Ostcam, Appl. Spectrosc.,40 (1986) 1184. T.A.Iscanderov, Ya.M.Kimelfeld, E.M.Smirnova, Chem.Phys.,ll2 (1987) 379.