V. K.
634
Kapur and 8.B. Wayland
Nitrogen4 4 Contact Shifts and Line Broadening Studies for Acetonitrile Complexes of Copper( I I), Nickel( I I), Cobalt(1I), and Titanium(1II) V.
K. Kapur and B. 8. Wayland*
Departmenf of Chemistry and Laboratory for Research on the Structure of Matter. University of Pennsylvania. PhiIadelphia Pennsylvania 79104 (Received September 28. 7972) Publication costs assisted by The University of Pennsylvania
Nitrogen-14 contact shifts and line broadening studies are reported for acetonitrile complexes of Cu(II), Ni(IX), Co(II), and Ti(II1). Acetonitrile nitrogen-14 hyperfine coupling constants for the series of metal ion complexes are Cu(I1) (30.0 1 2.0 MHz), Ni(I1) (27.0 f 2.00 MHz), Co(1I) (9.5 f 1.0 MHz), and Ti(II1) (negative). Opposite signs for the 14N coupling constant when the spin density is exclusively in the CT orbitals (Cu(I1) and Ni(I1)) or exclusively in n orbitals, Ti(III), provide direct evidence for positive anti negative contributions from a and x ligand spin density, respectively. Opposing contributions from CT and A spin density result in the small 14N coupling constants observed for Co(11)-, Fe(II)-, Mn(l1)-acetonitrile complexes. Kinetic data for psuedo-first-order ligand exchange for the Ni(I1) and Co(1T) complexes are evaluated from 14N line width studies.
Introduction Contact shifts of ligand nuclei are widely used to deduce the mechanism and magnitude of spin delocalization in paramagnetic metal ion comp1exes.l The preponderance of these studies have utilized proton nmr.2 Conclusions drawn from proton contact shifts are necessarily indirect for the-protons are not the ligand donor atoms. As part of a general investigation of the 14N contact shifts of nitrogen bound ligands, this paper reports on the 14N contact shifts and line broadening for acetonitrile complexes ofCu(II), Ni([I), Co(K11), andTi(II1). Experimental Section Materials Eastmaa acetonitrile was used for the preparation of complexes. It was dehydrated and purified by triple distillation over BaO, P205, and CaH2, respectively. The complexes of Cuz+, Ni2+, and Co2+ were prepared by Wickenden and Krause3 method, from the hydrated perchlorate salts obtained from the G. Fredrick Smith Chemical Co. Solutions for titanium(II1) studies were prepared by dissolving anhydrous Tic13 in acetonitrile. Sample Preparation. The samples for nmr were prepared such that the weight of each constituent (complex and acetonitrile solvent) and the total volume were known. All sample preparation and manipulations were carried out in a drybox under dry nitrogen or on a vacuum line. Solutions of complexes were further analyzed for the metal ion concentration, as an independent check on the solution composition. Solutions of metal complexes were degassed and sealed in 5-mm 0.d. thin-walled tubes along with a sealed degassed eapilliary tube of C H ~ N O Tas external standard. Sample tubes were sealed as close to the solution as possible while still permitting adequate expansion for higher temperature studies. A sample containing glare acetonitrile and a capillary of nitromethane was prepared as a reference and the nmr spectra were recorded at every temperatwe at which samples of metal complexes were studied. Several metal ion concentrations were studied for each complex. Apparatus. All I4bIspecta were run on a Varian HA-100 spectrometer using the I-iR mode, a t 7.22 MHz and using The Journal of Physical Chemistry, Vol. 77, No. 5, 7973
14N probe No. V-4333A. Variable temperature controller No. V-4341 was used to maintain constant temperature. The temperature was measured using a copper-constantan thermocouple and was found to be constant within 10.5"throughout most of the temperature range. The shifts were measured relative to nitromethane as an external standard.* The specta were recorded by sweeping both from high to low field and the reverse. Line width and contact shift values are averages of at least three runs a t each temperature. Spectra were recorded with the spectrometer in the HR and side-band modes. The rf power was kept much below the saturation level. The calibration of the spectra was done by side-band technique. T o expand the scale, the modulation frequency was changed from 2.5 to 1.5 kHz, which was accomplished by disconnecting the 2.5-kHz oscillator a t V-201 (in integrator decoupler unit) and' connecting an external oscillator with adjustable frequencies a t V-405 in the same box (V. 3521A). Analysis of Data. The basic equations for calculating the coupling constants and kinetics parameters are5 as follows Aw,/w = -A&,P,S(S f 1)h/g,,Pn3kT where A, i s the hyperfine coupling constant in Hz. AWobe.d
= p m A w m / [ ( z m / T,m
f
+
tm2AuW,,2]
where Pm = concentration of coordinated ligand us. total ligand concentration
where
T2
=
TzA is transverse relaxation time of
( ~ h - l ,
(1) G. A. W e b b , Annu. Rep. NMR (Nucl. Magn. Resonance) Soecfrosc.. 3, 211 (1970). (2) W . D. Horrocks. Jr.. and D. L. Johnston. Inom. Chem.. 10. 1835 (1971). (3) A. E. Wickenden and R . A. Krause, inorg. Chem.. 4, 404 11965) (4) M. Witanowski and H. Januszewski, J. Chem. SOC. 5. 1062 (1967) (5) T. J. Swift and R . E. Connick, J. Chem. Phys.. 3 7 , 307 (1962).
Acetonitrile Complexesof Cu(ll), N i ( l l ) , Co(ll),and Ti(lII)
635
TABLE I: Representative Data of Contact Shifts and Line Widths vs. Temperature for Cu(ll), N i ( l l ) , and Co(ll) Complexes _ I _ ~ _ - I - - _ _
.__ ll[Cu(Gti:3CN)rj] (CI04)p
1Q3/T3"K-'
- -104P, - ~
3.32 3.23 3.23 3.02 2.02 2.82 2.65 2.65
10.62 7.152 7."'[:!7
7.152 10.62 7.127
10.62 7.152
- A w / 2 ? r , klz
25
IO4/',
lO3/T,"K-'
102 65 63 61 77 50 72 47
2.08 2.13 2.18 2.23 2.28 2.28 2.34 2.40 2.46 2.46 2.59 2.59 2.69 2.73 2.82 2.91 3.02 3.31
a No line width data were collected for Cu(l1).
20
30 ( 0 3 1 ~
[Co(CH&N)61 (C104)z
[Ni (CH3CN) 6](CI04)2
- A w / 2 a , Hz
A U ,Hz ~
57 63 58
34 37 41 44 47
4.398 4.398 4.398 4.398 4.398 8.797 4.398 4.398 8.797 4.398 4.398 8.797 8.797 8.797 8.797 4.398 8.797 8.797
106 46 34 69 26 43 16 9
54 56 54 47 89 59 40 13 16 9
i03/T,"K-'
2.42 2.59 2.69 2.73 2.73 2.82 2.82 2.915 3.02 3.12 3.23 3.32 3.41 3.47 3.50 3.59 3.64
IO4 P,
19.91 19.91 19.91 9.499 6.638 4.691 6.638 9.499 4.691 9.499 4.691 9.499 4.691 4.691 9.499 4.691 9.499
- A w / 2 1 ~is the observed shift in Hz downfield from pure acetonitrile.
- A w / 2 1 ~ , Hz
Au, Hz
21 2 241 251 17 90 15 96 136 68 98 34 42
47 35 89 47 80
13
23 56 17 28
Au is full line width at haif-height.
35
OK-'
Figure 1. Contact shifts in radians per second normalized to P, = 1, for the solutions of acetonitrile complexes of Cu(ll), N i ( l l ) and Co(ll) plotted against 1/T. 0, N i ( l l ) (P, = 4.398 X and 8.797 x A , C u ( l l ) (Pm = 7.127 x 10-4, 7.157 x lo-*, and 10.62 X 0 , Co(ll) (P, = 4.691 X 9.4'99X lQ-'', and 19.9' X the pure solvent and AV is the observed full line width at half-height, The ef€ect of temperature on T , and thus on TZis given by 7, = ( h / k T )exp[(AHt/RT) - ( A S t / R ) ] where AH! and AS{ are the enthalpy and entropy of activation,
Results Cu(II) Complex. Nitrogen-14 contact shifts for [Cu(CH&N)e]( Cle)4)2 in neat acetonitrile are downfield relative to pure acetonitrile (Table I) which corresponds to a positive I*N coupling constant (+30 f 2 MHz). The contact shifts are found to be inversely dependent on temperature which demonstrates that the observed shifts are authentic contact shifts The limiting fast exchange region is obtained for the entire temperature range of our studies and therefore kinetic parameters were inaccessible from line broadening data.
1 0 ~ 1O K -~'
Figure 2. Log of transverse relaxation time normalized to P, = 1 for the solutions of acetonitrile complexes of N i ( l l ) and Co(ll) plotted against 1/T: 0, N i ( l l ) (P, = 4.398 X and 8.797 X l o w 4 ) ;0 Co(ll) (P, = 4.691 X and 9.499 X
Ni(II) Complex. Temperature dependence data for the i4N contact shifts and line widths for solutions of [Ni(CH&N)6](C104)2 in neat acetonitrile are found in Table I. Temperature dependence of the contact shifts and the line widths are plotted in Figures 1 and 2, respectively. Nitrogen-14 contact shifts for Ni(I1) solutions are downfield relative to free acetonitrile. The kinetics parameters from the line broadening are given in Table 11. The I4N coupling constant from line width data (25 f 2 MHz) is in satisfactory agreement with the value obtained directly from the contact shifts (27 2 MHz). Co(11) Complex. Nitrogen-14 contact shifts and line broadening data for solutions of [Co(CH&N)e](Cl04)2 in acetonitrile are found in Table I. Temperature dependence of the contact shifts and the line widths are plotted in Figures 1 and 2, respectively. Temperature dependent effects are found to be completely reversible. The i4N coupling constant evaluated from line broadening data is The Journal of Physical Chemistry, Vol. 77, No. 5, 1973
V. K. Kapur and E. E. Wayland
636 TABLE B S
TABLE Ill: TiCi3 in CH&N Contact Shifts vs. Temperature" _ _ I -
Ni(CHaCN)$+
CO(CH3CN)s2+
103/T,,"K-'
HZ
A Q , ~Hz
Shift (Au2
- Au2)
~
A n $ ,kcal
AS\,eu k (25'), sec-' A,, M H i
9.44 -7.8 1.45 X lo4 27.0
8.79 -4.2 2.7 x 105 9.5
found to vary with temperature from 7 to 10 MHz. The origin of this discrepancy in the analysis of the data is not yet clear. The I*N coupling constant obtained directly Erom contact shift measurements is 9.5 f 1 MHz. Kinetic parameters for ligand exchange of [Co(CH&N)6](C104)2 with bulk CH&N are given in Table 11. Ti(1II)Complexes Studies of Tic13 is acetonitrile were limited to a relative small temperature range because these solutions undergo an irreversible chemical change above 90". The observed nitrogen-14 shifts for solutions of 'Tic13 in acetonitrite are upfield relative to free acetonitrile, which is opposite to that observed for Cu(II), Ni(II), and Co(1I) solutions. The 14N coupling constant could not be obtained firom this study because the experiments could not be carried to sufficiently high temperatures to reach the limiting fast-exchange region. The increasing positive value of the shift (Table 111) with increasing temperature does, however, indicate that the I4N coupling constant is negative ( /A(14N) >> 6 MHz).
3.32 2.92 2.53
986 970 960
999 1007 1013
4-13 c 37 i.53
a Molality = 5.0429 X 14N shift of pure CH$N relative to nitromethane. C i4N shift of Tiat-acetonitrile solution relative to nitromethane.
of A(14N)/A(IH) be approximately constant. In satisfactory agreement with this expectation, the A(1*N)/A('H) ratio is 72 and 80 for the Cu(I1) and Ni(I1) complexes, respectively. Proton coupling constants used in this comparison of ratios were determined on the same solutions used in the 14N studies: Cu(I1) A(H) = 4.7 X lo5 Hz and Ni(1I) A(H) = 3.34 x 105 H ~ . The nitrogen-14 coupling constant for [Co(CH&N)6](C104)~(9.5 MHz) in acetonitrile is substantially smaller than those for Cu(1I) (30.0 MHz) and Ni(I1) (27.0 MHz) complexes. Octahedral cobalt(I1) (tzg5eg2) can directly delocalize spin in both the u and s ligand orbitals. The reduced A(14N) could result from a positive contribution from u spin density and negative contribution from the T spin density. The ratio A(14N)/A(IH) for the Co(I1) complex is 332, which is distinctly different from the ratio characteristic of u delocalization and su ports the presence of competitive (both u and T ) mechanisms. The still smaller values of 14N coupling constant (5.0 MHz) for Fe(I1)- 8 and The reported proton coupling constants for [M(3.2 MHz) for Mn(I1)-acetonitrile9 complexes also support (cH&N)6](C1(>4)2 (M = Ni(I1) and C O ( I I ) ) in ~ acetonithe presence of competitive mechanisms. trile, decrease by an order of magnitude in going from N$I) to Co(I1:. Octahedral Ni(I1) complexes tzg6eg2have In order to obtain direct evidence for the sign of the two unpaireid electrons in the d molecular orbitals which contribution to the A(I4N) from s spin density, solutions can transfer spin density to the u orbitals of the ligand. of Tic13 in acetonitrile were studied. The Ti(II1) complex While octahedral Co(l1) complexes tzg5eg2have unpaired is found to be in the slow ligand exchange region a t 30" electrons both in the tzg(n) and eg(u) orbitals, so that spin and elevated temperatures were required to observe sigdensity can directly reach both the g and R ligand orbinificant I4N shifts of the bulk CH&N. The observed 1,aiis. The very much smaller proton coupling constant for shifts and line widths are reversible up to -80". Above the Co(l1) complex has been proposed6 to result from 90" Ti(II1) probably disproportionates to Ti(I1) and Ti(1V) competitive spxn delocalization mechanisms. Spin density and irreversible changes occur. The data observed were in the u orbitals giving a negative coupling constant for insufficient to determine the value for A(14r\a); however, methyl protons and T spin density giving a positive conthe upfield I4N contact shifts corresponds to a negative tribution to A(H). The net negative A(H) value for u spin AP4N) coupling constant ( /A(14N)I >> 5.00 MHz). density was proposed to result from u-T correlation (negaThe observed negative 14N coupling constant for the tive contribution to A(H)). Spin delocalization in R ligand Ti(II1)-acetonitrile solutions, where the metal has only molecular orbital gives a positive coupling constant by diunpaired d, electron density, corroborates the contention rectly placing spin density in the methyl hydrogen Is orthat spin density transferred from the metal to the ligand bitals which make a substantial contribution to the R orin the 7: orbitals results in a negative contribution to bitals. A(I4N). The observed A(14N) for Co(II)-, Mn(I1)-, and Nitrogen-14 coupling constants for [Ni(C H ~ C N ) ~ ] ( C ~ O ~Fe(I1)-acetonitrile )Z complexes thus result from opposing CT and [Cu(CH3CN)o](C104)2 are positive. The direct deand T spin density contributions to ia(l4N).The negative localization of positive spin density to the acetonitrile u contribution to A(14N) from R spin density could result molecular orbitals and thus to the nitrogen 2s atomic orbital from either positive or negative spin density on the ligand results from molecular orbital formation with the metal eg with different mechanisms for transferring the effects to orbitals which contain the unpaired electrons. A 14N the 14N nucleus. Proton hyperfine coupling is important coupling constant of 4-30 MHz for the Cu(I1) complex in deciding this issue. Proton contact shifts in the Ti(II1) corresponds to a Nz8 spin density of +0.019. In calculating complex are downfieldlo relative to free acetonitrile correthe 2s spin density, we have used the relation f s = (2s)An/Azs rind the value of atomic hyperfine coupling (6) N. A. Matwiyoff and§. V. Hooker, Inorg. Chem.. 6, 1127 (1967). constant used fm Nzs is 1545 MHz.7 Both the Ni(I1) tzs6eg2 (7) D. R. Hartree, W. Hartree, and 8 . Swirles, Phi% Trans. Roy. SOC. and Gu(I1) tac6eg3 complexes have unpaired electrons London. Ser. A. 238, 229 (1940); D. R. Hartree and W. Hartree, Roc. Roy. Soc., Ser. A . 156, 45 (1936). Pxelusively in the o(eg) d orbitals. The mechanism of spin (8) R . J. West and S. F. Lincoln. Aust. J. Chem.. 24, 1169 (1971). delocalization to the GHJCN is therefore expected to be (9) W. L. Purcell and R. S. Marianelli, Inorg. Chem.. 9 , 1724 (1970) the same in both complexes, which requires that the ratio (10) C , C. Hinckley, Inorg. Chem.. 7, 396 (1968). The J O i J r n d of Phys;ca/ Chemistry, Vol. 77,
No. 5, 1973
Outer Sphere Complex between C0(en)3~+ and P043-
spondiiig to a positive spin density in the ligand T orbitals, The negative A(I*N) from positive i~ spin density must result from 7-(r spin correlation inducing negative spin density at the 141\1 nucleus. To our knowledge this is the first reported negative 14N coupling constant. The reported 14N coupling constant in NH3-b and nitrogen containing free radicals are all positive.ll Spin correlation in NH3+ places positive spin density in the Nzr3and negative in the N1, with the positive Nzs spin density dominating the coupling constant. In the Ti(III)--acetc,nitri~esystem either both N1, and Nzl contain negative irpin density or the N1, spin density dominates. Further theoretical work is needed to fully understand the oriigin of this negative 14N coupling constant. It i s of interest to note that similar trends have been observed in the 170coupling constant for aquo complexes.
637
In aquo complexes, the coupling constant changes sign from negative values of -5.5 X l o 7 for Cu(II)12 and -3.0 x 107 for Ni(II)13 to a positive value of 4.4 x 106 for Ti(II1) c0mp1ex.l~The signs of the 170coupling constants are reverse from that of 14N, because the gyromagnetic ratio of is negative.
Acknowledgment. The authors acknowledge the support of the National Science Foundation through Grant No. GH-33633and GP-28402.
(11)T.Cole, J . Chem. Phys.. 35, 1169 (1961). (12) W. B. Lewis, M. Alei, Jr., and L. 0. Morgan, J Chem Phys.. 44, 2409 (1966). (13) R. E. Connickand D. Fiat, J . Chem. Phys.. 44, 4103 (1966). (14) A. M. Chmelnickand D. Fiat, J. Chem. Phys.. 51, 4238 (1969)
Outer Sphere Complex between Trisethylenediaminecobalt( I I I) and Phosphate Thomas H. Martin and B. M. Fung*l Deparlment of Chemistry, Tufts University, Medford, Massachusetts 02155 (Received September 14, 7972) Publications costs assisted by the National Science Foundation
59f.c:~
resonance of C0(en)3~+(en = ethylenediamine) in the presence of phosphate and 31P resonance of phosphate in the presence of Co(ex1)3~+were studied a t different salt concentrations and pH's. The equilibrium quotient for the formation of outer sphere complex was calculated from the 5 9 C chemical ~ shift data and was found to be 12.0 f 2.0 M-I a t ionic strengths around one. 'This value fits the 3lP chemical shift data very well. The pH dependence of the 59C0 shift is also studied.
Nuclear rnagnetic resonance (nmr) is a useful method €or the study of the second coordination sphere of transition metal complexes. A number of reports on that subject has appeared in recent year^.^-^ The formation of an outer sphere complex usually occurs between a transition metal complex and a solvent molecule or another anion. The outer sphere complex between C0(en)3~+(en = ethylenediamine) and Po43-is of particular interest because of its possible stereo specificity due to hydrogen bondinglO and its effect on Lhe conformation of the chelate rings in C ~ ( e n ).5~ ~ Its + formation has been studied by circular dichroism10-12 and proton nmr.4-6 However, to our knowledge, the equilibrium constant has not been determined, although that between C0(en)3~+and other anions has been reported.?lJ3J4 In this paper we report the study of outer sphere complex between Co(en)s3+ and - by 59C0 and 31P nmr with the determination of the equilibrium quotient. The choice of 5gC0and 31Pas the probing nuclei in the nmr study is based upon the sensitivity of their chemical shifts to a change in the environment. It is well known that diamagnetic Co(II1) complexes show extremely large 59C0 shifts due to the existence of low-lying excited electronic states.lS Very small changes, such as the solvent composition in trisa~atylacetonatocobalt(III)~ and isotope substitution in C0(en)3~+,5 cause appreciable variation in
the 59C0 chemical shift. The magnitude of the change of 31P chemical shift with the environment is much smaller but can be measured more accurately because 31P is a spin 1/2 nucleus, and therefore has much sharper nmr lines. The change of 31P resonance in phosphate and polyphosphate ions due to ion-pair formation with alkali metal cations has been reported.16 The proton resonance of the (1) Present address: Department of Chemistry, University of Oklahoma, Norman, Okla. 73069. (2) B. M. Fung, J. Amer. Chem. SOC.,89,5788 (1967). (3) B. M. Fungand I . H. Wang, Inorg. Chem., 8,1967 (1969). (4) J. L. Sudmeier and G . L. Blackmer, J. Amer. Chem. SOC.,92,5238 119701. -, >
~
(5) J. L. Sudmeier, G . L. Blackmer, C. H. Bradley, and F. A. L. Anet, J. Amer. Chem. SOC.,94,757 (1972). (6) L. R. Froebeand B. E. Douglas, Inorg. Chem., 9,1513 (1970). (7) D. R. Caton, Can. J. Chem., 47, 2645 (1969). (8) L. S.Frankel, J. Phys. Chem., 73, 3897 (1969). (9) L. S. Frankel, C. H. Langford, and T. R . Stengle, ,/. Phys. Chem., 74, 1376 (1970). (IO) S. F. Masson and B. J. Norman, Proc. Chem. Soc., London, 339 J. Chem. SOC.A, 307 (1966). (1964); (11) R. Larssoti, S. F. Masson, and B. J. Norman, J. Chem. Soc. A , 301 (1966). (12) H. L. Smith and B. E. Douglas, Inorg. Chem,, 5, 784.(1966). (13) F. A. Poseyand H.Taube,J. Amer. Chem. Soc., 78.15 (1956). (14) M. G. Evans and G. N. Nancollas, Trans. Faraday Soc., 49, 363 (1953). (15) J. W. Emsley, J. Feeney, and L. H. Sutcliffe, "High Resolution Nuclear Magnetic Resonance Spectroscopy." Pergarhon Press, Oxford, 1966,p 1178ff. (16) M. M. Crutchfiold and R. R. Irani, J. Amer. Chem. Soc., 87, 2815 (1 965).
The Journal of Physical Chemistry, Vol. 77, No. 5, 1973
