W. H. CANTY and G. K. WHEELER R. T. Vanderbilt Co., Inc., East Norwalk, Conn. R. R. MYERS Lehigh University, Bethlehem, Pa.
I,IO-Phenanthroline
Drier Catalyst Activity in Organic Coatings Drying data on commercial alkyds reveal the presence of metal complexes and tell what chemical species is the most active drier
h
A catalyst, 1,lO-phenanthroline, has been used for a number of years by DRIER
. )
the protective coatings industry.
Literature Background Peroxide build-up in presence of tris(lJ0-phenanthroline) cobalt(111)
Decomposition of peroxy cornpounds by amine activators Unusual activity of 1,lO-phenanthroline with manganese salts Optimum ratio of 1,IO-phenanthroline to metal gives maximum acceleration Catalysis of linseed oil by aminemetal catalysts Cobalt salts preferred with 1,lOphenanthroline in emulsion systerns
(16) (3,4, 8, 1b
(16)
(13) (10) (14)
I n the work described here, the mechanism was investigated by determining the complex species formed when the amine and metal are combined in an oleoresinous system. The data are not scientifically exact, but the conclusions appear valid. Determination of Complex Species
Test Procedures. A commercial, medium-oil, soybean alkyd having an acid value of 5.7 and containing phthalic anhydride (30 to 4001,) was chosen for this work. An infrared curve of this vehicle indicated that it was practically identical to Aroplaz 1085-M-50 (ArcherDaniels-Midland) with the exception that the former contained some xylene as solvent. Unpigmented films of this alkyd resin solution were examined for drying rates on a Gardner Drygraph (5)operated in an air-conditioned room maintained a t 75' F. and 50% R.H. Films were applied on plate glass using a 4-mil clearance applicator blade which delivers a wet film 2.2 mils thick. All mixtures were aged for 1 day before drying tests were made. Two series of mixtures were prepared, one with manganese metal and one with
1 Both
cobalt and manganese form complexes. A single complex species with coordination number of 4 appears to be formed with cobalt; an equilibration of two species is indicated with manganese.
1 A fairly constant ratio of bound amine to metal is maintained over a wide range of metal concentrations. This ratio is accompanied by a nearly constant concentration of free amine. b
Pigmentation with titanium dioxide or carbon black does not alter the ratio but has a slight effect upon free amine concentration.
1 The ratio of
bound amine to metal is the determining factor in the drier catalyst activity of a 1,IOphenanthroline. However, each vehicle and pigment has a certain demand for amine which determines the ratio of total amine to metal that must b e added to a given system for optimum results.
1 The activated intermediate in the catalysis of oxidative polymerization appears to be formed with less expenditure of energy when 1,IO-phenanthroline is present. cobalt metal. Each series was comacid. Manganese and cobalt were added posed of four stock mixtures containing as (5% metal naphthenates (by weight). 0.001, 0.005, 0.02, and 0.08% metal, Similarly, two pigmented mixtures respectively, based on the weight of were prepared, one containing carbon alkyd solids. To aliquots of each of black (0.2 pound per gallon) and the these stock mixtures 1,lo-phenanthroother titanium dioxide (3.0 pounds per line (available in solution as Activ-8, gallon) and containing manganese naphR. T. Vanderbilt Co., Inc., New York) thenate as the metallic drier. I n these was added in varying concentrations cases the drier was tested only a t the to give molar ratios of amine to metal three highest concentrations. ranging from 0.25 to 1.O u p to 20.0 to 1.O Results. For each concentration of by weight. I n preparing the above metal, an optimum ratio of 1,lO-phenanthroline to metal produced a minimum mixtures, 1,lO-phenanthroline was added as a 38% solution (by weight) in a sol(fastest) drying time. From these curves the total moles of 1,lO-phenanthroline vent system consisting of two parts of were calculated (Tables I and 11). butanol to one part of 2-ethylhexoic /--
----.
F' )T
% \
JANUARY 1960
67
M O L A R R A T I O IJO-PHENANTHRCLINE
MOLAR RATIO I, io-PHENANTHROLINE
For each concentration of metal, an optimum ratio of 1,lO-phenanthroline produced a minimum drying time
T h e actual species formed at optimum ratios, when 1,lO-phenanthroline and metal are combined in the vehicle, were determined as follows. The objective is to utilize an equation showing the extent of complexing and the concentration of free amine present. T h e classical method of Bjerrum (7, 7) was employed for the determination of ii, the average number of moles of 1,lO-phenanthroline (ligand) bound per mole of metal ion present. The total amine added, CA (moles per liter), is distributed between free amine ( A ) and bound amine ACM, where ii is the average complex species formed :
CA = ( A )
The concentration of free (unbound) amine ( A ) a t equilibrium is found by substituting the value A = 2.1 in Equations l through 4 giving, respectively, ( A ) = 0.0018, 0.0021, 0.0019, and 0.0013 mole per liter. Calculation of fi for Manganese. ii for manganese was determined in a similar manner, giving a value of 1.4 in all cases. The concentrations of free amine were 0.0016, 0.0012, 0.0014, and 0.0009 mole per liter.
Table I. Amine Optima a t Various Cobalt Concentrations
+ ZCM
Empirical data from drying time measurements give 1,lO-phenanthroline-cobalt optium ratios
cos %
1,lO-Phenanthroline Concn. (Ca)," Mole/Liter
1,lO-Phenanthroline/ Co, Optimum Ratiob
Co Concn. (C.\r), Mole/Liter
0.080 0.020 0.005 0.001
0.0302 0.00928 0.00369 0.00169
2.24 2.73 4.34 9.92
0.0135 0.00340 0.00085 0.00017
or
Values for CA and CM are taken from Tables I and 11. By the use of simultaneous equations, A and ( A ) were calculated from the data for the four metal concentrations used. Calculation of ii for Cobalt. A for cobalt was determined using the following simultaneous equations. The equations were capable of simultaneous solution, justifying the procedure used. [0.0302 - ( A ) ] 0.0135
(1 1
[0.00928 - ( A ) ] 0.00340
(2)
A = [0.00369 - ( A ) ] 0.00085
(3)
A = [0.00169 - ( A ) ] 0.00017
(4)
fi=
0.00928 - (0.0302 - 0.0135 A ) 0.00340
At optimum ratio.
Table II.
From figure at top left.
Amine Optima a t Various Manganese Concentrations
Empirical data from drying time measurements give 1,lO-phenanthroline-manganese
hfn,
%
0.080 0.020 0.005 0.001 a
At optimuni ratio.
1,10-Phenanthroline Concn. ( C A ) , ~ Mole/Liter
1,lO-Phenanthrolinej Mn, Optimum Ratiob
0.0219 0.00626 0.00264 0.00117
1.74 2.90 6.50
INDUSTRIAL AND EW
1.51
optimum ratios
Mn Concn. (CM), Mole/Liter 0.0145 0.00360 0.00091 0.00018
From figure at top right.
Pigmentation Did Not Alter the Amine to Metal Ratio for Manganese a t Optimum Drying Time 1,lO-Phenanthroline 1,lO-Phenanthroline/ Mn Concn., Concn. (CA), Mn, Optimum Mn, % Mole/Liter Ratio (CM), Mole/Liter Carbon Black 0.080 0.020 0.005
0.0250 0.0089 0.0045
0.080 0.020 0.005
0.0240 0.0059 0.0025
2.1
Similarly, from Equations 3 and 4, ii = 2.1. T h e average of these three values is li = 2.1.
68
a
Table 111.
From Equation 1, ( A ) = 0.0302 0.0135 A Substitution of ( A ) in Equation 2 gives
ii =
T h e near constancy of free amine ( A ) justifies the use of simultaneous equations in solving for A and ( A ) . Results for Pigmented Mixtures. The data in Table I11 were utilized in the calculation of fi in a manner similar to the two previous sections. Values of f = 1.5 and ii = 1.6 were obtained for the carbon black and titanium dioxide pigmented systems, respectively. Corresponding values for ( A ) were 0.0030 and 0.0020.
1.72 2.47 4.95
0.0145 0.0036 0* 0009
Titanium Dioxide 1.65 1.65 2.74
0.0145 0.0036 0.0009
DRIER C A T A L Y S T A C T I V I T Y Discussion A
bis( 1,lO-phenanthroline) cobalt complex is formed a t the 1,lO-phenanthroline concentration which produces most rapid drying. The implication is that the single complex species (bis-) far outweighs the other species (mono, tris) in the equilibrium mixture. An important aspect of the drying time curves is that the sensitivity of drying time to 1,lOphenanthroline concentration is much greater in the case of cobalt than with manganese-i.e., enhancement of cobalt metal activity is obtained only within a narrow range of 1 ,lo-phenanthroline concentrations. This does not hold true in modified latex emulsion coatings, however (74). Maximum acceleration of manganese catalysis is obtained a t an amine to metal ratio of 1.5 to 1.O indicating that an equilibrium mixture of two amminemetal species is formed. Either of the following 50 to 50 mixtures of species will give an fi of 1.5 : Uncomplexed manganese and tris(1.1 0-ahenanthroline) manganese. Mono(1,lO-phenanthroline) and bis(1 ,lo-phenanthroline) manganese. Because it is difficult to visualize the existence of 50% free, uncomplexed manganese in the presence of an excess of 1,lO-phenanthroline, and because there are only six coordination positions ardund manganese, the second alternative appears more likely. It is interesting that pigmentation does not alter the molar ratio of bound amine to metal (5) for manganese a t the point of optimum drying time. The fact that fi is not integral may explain why the activity of manganese is increased by 1,lo-phenanthroline to a somewhat greater extent than that of cobalt. Bjerrum (7) points out that half-integral E values can be taken as evidence of an equimolar mixture of the two neighboring species of integral amine content. This equilibration is believed responsible for the synergistic action between 1,lO-phenanthroline and manganese, as one species is a strong oxidizer while the other is a reducer. This observation can best be explained on the basis of the electronic configurations of the various species. The customary way of showing the electronic configuration of an element is by the atomic orbital picture; for example, cobalt is 3d74s2 and is shown schematically as
valent cobalt ion or of covalently bound cobalt with an oxidation state of two:
a t present, the authors visualize the possibility of forming such inner orbital configurations, in aprotic media, in the case of the mono- and bis(1,lO-phenanthroline) coordination compounds. When true complexing occurs, these configurations are disturbed by the entry of electron pairs from the amine. If one molecule of 1,lO-phenanthroline enters the inner coordination sphere of a cobalt ion, the configuration is
Go++ ion
.. .. . . .
- - - - -
- - - -
Co + + coval.
.. -.. -. -. -.
-
3d
.o
.O
- - - 4s
4P
where the circles represent electrons from the bonded partner. The ionic representation is conventional. The covalent one is still in the speculative stage, in which the union of cobalt with two negative atoms is pictured as taking place with the involvement of two orbitals: the 4s, which supplied both electrons, and the 4p, to which one of the electrons was promoted in the course of oxidation.
Go (1,lO-phenanthroline)+ +
.. -.. -.. -
x x
__
x x _-
. ---~
3d
4s 4p where x represents an electron from nitrogen (1,lO-phenanthroline has two nitrogen atoms). Other inner cobalt complexes would have the structures:
Y
.
'
I
Co(l,lO-phenanthroline)t++
Co(1,lO-phenanthr~line)~ ++
.. .. .. x x .. .. x x e .
x x
x x -
x x
x x --
x x
*
x x x x
x x
____--
~
3d
4s
4P
*
__ 5s
I
The picture presented here allows the oxidation state of the metal to be portrayed just as unequivocally as if two electrons had been lost by ionization. The small circle represents bonding to an element on the electronegative side of the metal in question; this type of bonding
x x __ __
-_
*
Mn (l,lO-phenanthroline)z++
-* . -* *_ x_ x_ x_x _x_x - -
Mn (1,lO-phenanthro1ine)afc
-
"
*
'
X
bis( 1,lO-phenanthro1ine)-
that the species is not a strong reducer. The other two species have single s electrons which are easily lost irreversibly. I n the case of manganese (3d54s2) the possible species are :
- -
X
x x
* * " X X
-
x x
x x
x x
--
--
- - - -
-
x x
x x
-
~
3d
is an oxidation in the general sense of the word. The interpretation which follows represents a departure from current concepts of the orbitals involved in the formation of cobalt ion and manganese ion phenanthroline complexes in aqueous media. It is recognized that coordination in aqueous media generally involves outer orbitals until the point is reached a t which penetration of the inner orbitals can take place all a t once. While there is little evidence to support these views
t I
Mn (1,lO-phenanthroline)z++
4#
4s
Here divalent manganese alone is not as good a catalyst as divalent cobalt because the 3d6 configuration is harder to break u p than the 3d7 configuration of cobalt (70); but the combination of the mono- and bis(1,lo-phenanthroline) complexes offers simultaneously a strong oxidizer (mono) and reducer (bis). Because of the lability of the coordinated amines, there is an incessant interchange of amine among the manganese nuclei. Therefore, a reversible cycle can occur between the complex species and oxygen:
+ox _ . -
- 1,lO-phenanthroline Divalent cobalt is different. The configuration is either that of the di-
the
Mn (1,lO-phenanthroline)++
Mn (1,lO-phenanthroline)++ co
Only
Go++ complex has an electron so situated
Mn (1,lO-phenanthroline) O x + +
--f
+ 1,lO-phenanthroline
-ox +--
I
J.
Mn (l,10-phenanthroline)2O x + +
VOL. 52, NO. 1
JANUARY 1960
69
where O x represents any electronegative element, probably molecular oxygen. The mono complex picks u p oxygen, then adds another 1,lO-phenanthroline group which reduces its affinity for oxygen so that the Mn(1,lO-phenanthroline)zT- species rapidly forms. Single headed arrows are used to show the proposed route. not to imply that each step is not an equilibrium step. Operation of the cycle may be reasoned on the basis of electronic configurations using the criteria of favored configurations ( 9 ) and spatial symmetry ( 7 7) :
spatial stability and electronic instability (Table IV). The equilibration that gives rise to ii = 1.5 lies between the species MnA++ and iMnA2+-, of which the monoammine is the oxygen-gathering moiety. The combination is composed of unstable forms at all times, either spatially or electronically. When oxygen is attached to either complex ion, the result is an odd number of bonds; when the ion is unoxidized. the result is an electronic deficiency or excess.
4s
C.N. Hybrid
411 of the uncomplexed amine is not necessarily "free" amine. I t is possible that some of the free amine is used to counteract the natural antioxidants in the oil or to coordinate with metal contaminants. This phenomenon of a minimum drying time for 1,lO-phenanthroline in the absence of added metal is the exception, rather than the rule, for vehicles examined in this laboratory. The amount of free amine is slightly higher in the case of carbon black. This increase in the amount of free amine ( A ) in going from unpigmented to titanium dioxide to carbon black pigmented systems may reflect a demand by the pigment for amine. Adsorption of amines by carbon black has hem demonstrated frequently (2, 77), and the high energy surface of titanium dioxide (6) is also capable of immobilizing some of the added 1,lO-phenanthroline. Acknowledgment
The authors thank RIilo Stearns for his generous advice and assistance and Patricia Carroll and George O'Brien for help in preparing this manuscript.
A = l,l0-phenanthroline:
literature Cited
Ox = oxygenated form, probably 3In-0-0. l\InAtT* = excited state, C.N. = coordination number. The d3s configuration for MnA21-+ appears to be in conflict Ivith the d3sp of MnA,Ox'+, but it is not. The degeneracy of the d3sF h>-brid permits the oxygen electron to leave from a p orbital rather than from the d orbital in which it was originally found. TVhereas the monoammine is electronically deficient (tends to acquire 10 electrons in the d shell), the oxidized form is spatially unsatisfied (tends to a coordination number of 4 or 6 ) . The acquisition of one additional amine elevates the coordination number IO 5; and as a result this species is also unstable. Reversion to h l n ,42'+ by loss of the oxygen (which is re2ponsible for the odd coordination number) gives a structure which resembles the monoammine in
Table IV. Balance between Electronic and Spatial Characteristics of Complex Species"
1,lO-Phenanthroline alters orbital conflguration of manganese via complex formation Electronic Spatial Bond Character- CharacSpecies Hybrid istics teristics MnA-+ d2 Deficiency Even MnAOx" d3 Stable Odd MnA?Ox+& d 3 s p Stable Odd MnAz A dqs Excess Even MnA+-" d2 Deficiency Even a 4 = 1,lO-phenanthroline; Ox = oxygenated form, probably RIn-0-0; M n A + + * = excited state.
70
INDUSTRIAL AND ENGINEERING CHEMISTRY
These considerations shoiv that the simple picture of Mn.42++ as a reducer (because of its p1 configuration) and MnA-' as oxidizer (because of d 9 ) is close enough to the more involved concept to be substituted for it when the situation \vanants. It would be incorrect to say that the two species oxidize and reduce each other. Therefore, the addition of 1,I@-phenanthroline increases the efficacy of manganese as a catalyst by forming a complex in which electrons are shifted from one orbital to another with minimum expenditure of energy, so that the possibility of finding a species in an excited (active) electronic state is materially increased. I n the case of cobalt, complexing with l,l@-phenanthrolineresults in the formation of bis(1 ,IO-phenanthroline)Co'+ Tvhich is neither a strong oxidizer nor reducer. Cobalt, therefore, would be expected to be less responsive than manganese to the action of l,l0-phenanthroline. This is in accordance with previously published observations (70). Having considered the mode of activity of the complexed portion of l,l@-phenanthroline, attention is now directed to its activity alone. A series of mixtures containing no metallic driers set most rapidly when an optimum concentration of 0.0028 mole of amine per liter of alkyd was employed. This figure is larger than the free, unbound amine ( A ) determined from the metal-catalyzed formulations ('4 = 0.0009 to 0.0021) but is of the same order of magnitude.
(1) Bjerrum, J.,"MetalAmmine Formation in Aqueous Solution," P. Haase and Son, Copenhagen, 1941. (2) Damusis, A. (to Sherwin Williams Co.), U. S.Patent 2.816.046 (Dec. 10. 1957). (3) Embree, W.'H., 'Spolsky, R., krilliams, H. L., IND.EKG.CHEM.43, 2553 (1951). (4) Fordham, J. W. I,., Williams, H. I,., Can. J . Research 28B, 551 (1950). (5) Gardner, H. A , , "Physical and Chemical Examination of Paints, Varnishes, Lacquers, and Colors," 11th ed., p. 152, H. A. Gardner Laboratory, Bethesda, Md., 1950. (6) Healey? F. H., Chessick, J . J.: Zettlemoyer, A . C., Young, G. T., J . Phys. Chem. 58,887 (1954'1. (7) Martell, A . E., Calvin, hl., "Chemistry of the Metal Chelate Compounds," p. 78, Prentice-Hall, Sew York, 1952. (8) Mitchell, J. M.: Spolsky, R., Williams, H. L., ISD. ENG.CHEM.41, 1592 (1949). (9) Moeller, T., "Inorganic Chemistry," Wiley, Kew York, 1952. (10) Myers, R. R., Zettlemoyer, A. C., IND. ENG.CHEM.46, 2223 (1954). (11) Sidgwick, N. V.: "The Electronic Theory of Valence," 1st ed., Oxford Univ. Press, Oxford, England, 1947. (12) Spolsky, R., Williams, H. L., IND. ENG.CHEM.42, 1847 (1950). (13) Stearns, M. E., Q f i c . Dig. Federntioq Paint 3 Varnish Production Clubs 26, 817 (1954). (14) Stearns, M. E., Cantv, Mi'. H., Ihzd.. 31). -ix - 119~1. (lsf Wheeler, G. K. (to R. T. Vanderbilt Co.. Inc.), U. S. Patent 2,526,718 (Oct. 24, 1950); 2,565,897 (Aug. 28, 1951). (16) 'Ilrorthington, E. A . , Nicholson, D. G., Paint, Oil Clem. Rec. 112, 20 (1949). (17) Zettlemoyer, A. C.: Schaeffer, W. D., Sewman, G. H., Division of Colloid Chemistry, 131st Meeting, ACS, Miami, Fla.: April 1957. 7
\ - _ - -
RECEIVED for review .4pril 29, 1959 ACCEPTEDAugust 27, 1959 Division of Paint, Plastics, and Printing Ink Chemistry, 135th Meeting: ACS, Boston. Mass., .April 1959.
