Strengthening of N-H?Co hydrogen bonds upon increasing the basicity of the hydrogen bond acceptor (Co)

Article abstract of DOI:10.1021/om950777e

Low temperature crystal structures of (DABCO)H⁺Co(CO)₄^- (1) and (DABCO)H⁺Co-(CO)₃PPh₃^- (2) (DABCO = 1,4-diazabicyclooctane) indicate that both salts exhibit N-H? Co hydrogen bonding. IR and NMR data indicate that these hydrogen bonded species persist in nonpolar solvents such as toluene, but exist as solvent separated ions in more polar solvents. Replacement of the axial CO ligand by PPh₃ leads to a shortening of the N?Co separation in the solid state from 3.437(3) to 3.294(6) ?. This change is accompanied by an increase in the angle between the equatorial carbonyl ligands. Thus, the crystallographic results suggest a strengthening of the N-H?Co hydrogen bond upon increasing the basicity of the metal center, the first observation of this type in the solid state. This assertion is supported by variable-temperature ¹H and ¹³C NMR data in toluene-d₈ solution which, discussed in the light of ab initio calculations, indicate that the barrier to a fluxional process involving cleavage of the N-H?Co hydrogen bond is greater in 2 than in 1. The crystal structures of 1 and 2 have been determined by X-ray diffraction at 135(5) and 123(5) K, respectively [1 monoclinic, P2₁/n (No. 14), a = 8.728(2), b = 23.333(5), c = 12.146(2) ?, β= 95.74(2)°, V = 2461.1(9) ?³, Z = 8, R(F) = 0.043, R_w(F) = 0.043, S(F) = 1.21: 2 orthorhombic, Pca2₁ (No. 29), a = 16.084(8), b = 8.874(3), c = 17.312(3) ?, V = 2471(1) ?³, Z = 4, R(F) = 0.065, R_w(F) = 0.060, S(F) = 1.16].

Full text of DOI:10.1021/om950777e

Organometallics 1996, 15, 1441-1445
1441
Str en gth en in g of N-H‚‚‚Co Hyd r ogen Bon d s u p on
In cr ea sin g th e Ba sicity of th e
Hyd r ogen Bon d Accep tor (Co)
Dong Zhao, Folami T. Ladipo, J anet Braddock-Wilking, and Lee Brammer*
Department of Chemistry, University of MissourisSt. Louis, 8001 Natural Bridge Road,
St. Louis, Missouri 63121-4499
Paul Sherwood*
Theory and Computational Science Division, CLRC Daresbury Laboratory,
Daresbury, Warrington WA4 4AD, UK
Received October 2, 1995X
Low temperature crystal structures of (DABCO)H+Co(CO)4 (1) and (DABCO)H+Co-
-
-
(CO)3PPh3 (2) (DABCO ) 1,4-diazabicyclooctane) indicate that both salts exhibit N-H‚‚‚
Co hydrogen bonding. IR and NMR data indicate that these hydrogen bonded species persist
in nonpolar solvents such as toluene, but exist as solvent separated ions in more polar
solvents. Replacement of the axial CO ligand by PPh3 leads to a shortening of the N‚‚‚Co
separation in the solid state from 3.437(3) to 3.294(6) Å. This change is accompanied by an
increase in the angle between the equatorial carbonyl ligands. Thus, the crystallographic
results suggest a strengthening of the N-H‚‚‚Co hydrogen bond upon increasing the basicity
of the metal center, the first observation of this type in the solid state. This assertion is
supported by variable-temperature 1H and 13C NMR data in toluene-d8 solution which,
discussed in the light of ab initio calculations, indicate that the barrier to a fluxional process
involving cleavage of the N-H‚‚‚Co hydrogen bond is greater in 2 than in 1. The crystal
structures of 1 and 2 have been determined by X-ray diffraction at 135(5) and 123(5) K,
respectively [1 monoclinic, P21/n (No. 14), a ) 8.728(2), b ) 23.333(5), c ) 12.146(2) Å, â )
95.74(2)°, V ) 2461.1(9) Å3, Z ) 8, R(F) ) 0.043, Rw(F) ) 0.043, S(F) ) 1.21; 2 orthorhombic,
Pca21 (No. 29), a ) 16.084(8), b ) 8.874(3), c ) 17.312(3) Å, V ) 2471(1) Å3, Z ) 4, R(F) )
0.065, Rw(F) ) 0.060, S(F) ) 1.16].
In tr od u ction
metal hydride moieties can also serve as hydrogen bond
acceptors, i.e. X-H‚‚‚H-M.8 In both cases N-H or
O-H groups typically function as the hydrogen bond
donor.
In recent years there has been considerable interest
in hydrogen bonding in which transition metal centers
are involved. A number of groups have described
examples of hydrogen bonding in which an electron-rich
transition metal center acts as the hydrogen bond
acceptor, i.e., X-H‚‚‚M.1-6 Work to date in this area
has been summarized in two very recent reviews.1d,7
Recent work has also demonstrated that transition
A hydrogen bond, X-H‚‚‚Y, can in effect be considered
as a competition for a proton between two bases, X and
Y. When one base (X) is substantially stronger than
the other, the proton will be bonded to that base via a
σ-bond, X-H, while forming a weaker, often largely
electrostatic, interaction with the other base. If the
strength of the weaker base (Y) is increased, the
hydrogen bond becomes stronger and shorter. Thus, the
distances H‚‚‚Y and X‚‚‚Y should decrease. A concomi-
tant weakening and lengthening of the covalent interac-
tion X-H is also anticipated. Increasing the basicity
of Y ultimately leads to proton transfer to give an H-Y
covalent bond. The extent to which a (reversed) X‚‚‚H-Y
hydrogen bond is then formed depends upon the polarity
X Abstract published in Advance ACS Abstracts, February 15, 1996.
(1) (a) Brammer, L.; Charnock, J . M.; Goggin, P. L.; Goodfellow, R.
J .; Orpen, A. G.; Koetzle, T. F. J . Chem. Soc., Dalton Trans. 1991,
1789. (b) Brammer, L.; McCann, M. C.; Bullock, R. M.; McMullan, R.
K.; Sherwood, P. Organometallics 1992, 11, 2339. (c) Brammer, L.;
Zhao, D. Organometallics 1994, 13, 1545. (d) For a short review see,
Brammer, L.; Zhao, D.; Ladipo, F. T.; Braddock-Wilking, J . Acta
Crystallogr. 1995, B51, 632. (e) Brammer, L.; McCann, M. C.; Bullock,
R. M.; McMullan, R. K.; Sherwood, P. Unpublished results. For
structure of Et3NH+Co(CO)4 at 15 K see ref 1b.
-
(2) (a) Wehman-Ooyevaar, I. C. M.; Grove, D. M.; Kooijman, H.; van
der Sluis, P.; Spek, A. L.; van Koten, G. J . Am. Chem. Soc. 1992, 114,
9916. (b) Wehman-Ooyevaar, I. C. M.; Grove, D. M.; de Vaal, P.; Dedieu,
A.; van Koten, G. Inorg. Chem., 1992, 31, 5484.
(3) Kazarian, S. G.; Hamley, P. A.; Poliakoff, M. J . Am. Chem. Soc.
1993, 115, 9069.
(4) (a) Albinati, A.; Lianza, F.; Pregosin, P. S.; Mu¨ller, B. Inorg.
Chem. 1994, 33, 2522. (b) Albinati, A.; Lianza, F.; Mu¨ller, B.; Pregosin,
P. S. Inorg. Chim. Acta 1993, 208, 119.
(5) (a) Shubina, E. S.; Krylov, A. N.; Timofeeva, T. V.; Struchkov,
Yu. T.; Ginzburg, A. G.; Loim, N. M.; Epstein, L. M. J . Organomet.
Chem. 1992, 434, 329. (b) Shubina, Ye. S.; Epstein, L. M. J . Mol. Struct.
1992, 265, 367.
(6) Calderazzo, F. Fachinetti, G. Marchetti, F. Zanazzi., P. F. J .
Chem. Soc., Chem. Commun. 1981, 181.
(8) (a) Lee, J . C.; Peris, E.; Rheingold, A. L.; Crabtree, R. H. J . Am.
Chem. Soc. 1994, 116, 11014. (b) Peris, E.; Lee, J . C.; Rambo, J . R.;
Eisenstein, O.; Crabtree, R. H. J . Am. Chem. Soc. 1995, 117, 3485. (c)
Peris, E.; Wessel, J .; Patel, B. P.; Crabtree, R. H. J . Chem. Soc., Chem.
Commun. 1995, 2175. (d) Wessel, J .; Lee, J . C., J r.; Peris, E.; Yap, G.
P. A.; Fortin, J . B.; Ricci, J . S.; Sini, G.; Albinati, A.; Koetzle, T. F.;
Eisenstein, O.; Rheingold, A. L.; Crabtree, R. H. Angew. Chem., Int.
Ed. Engl. 1995, 34, 2507. (e) Park, S.; Ramachandran, R.; Lough, A.
J .; Morris, R. J . Chem. Soc., Chem. Commun. 1994, 2201. (f) Lough,
A. J .; Park, S.; Ramachandran, R.; Morris, R. J . Am. Chem. Soc. 1994,
116, 8356. (g) Stevens, R. C.; Bau, R.; Milstein, D.; Blum, O.; Koetzle,
T. F. J . Chem. Soc., Dalton Trans. 1990, 1429.
(7) Canty, A. J .; van Koten, G. Acc. Chem. Res. 1995, 28, 406.
0276-7333/96/2315-1441$12.00/0 © 1996 American Chemical Society

1442 Organometallics, Vol. 15, No. 5, 1996
Zhao et al.
CH2); 10.1 (s, NH). 13C NMR (75.3 MHz, C6D5CD3, 298 K) δ
47.2 (s, CH2), no signal observed for CO ligands; (75.3 MHz,
C6D5CD3, 188 K) δ 43.9 (s, CH2) no signal observed for CO
ligands. IR (CH3CN) ν CO 1892 (s) cm-1; IR (C6H5Me) ν CO
of the H-Y bond and upon the basicity of X. Often
dissociation to yield X + H-Y will occur.
Solution IR studies by Poliakoff and Kazarian,3 and
by Epstein and co-workers,5 on hydrogen bonds from
alcohols to d8 Co, Rh, and Ir centers, and to d6 Fe, Ru,
and Os centers, respectively, indicate that O-H‚‚‚M
hydrogen bonds are strengthened by increasing the
basicity at the metal center. Protonation of the most
basic metal centers was observed, but there was no
evidence for (reverse) M-H‚‚‚O hydrogen bond forma-
tion. Indeed, observations of M-H‚‚‚O hydrogen bonds
have been rare to date. Epstein and co-workers9 first
suggested, in 1993, that such an interaction is present
in the system (η5-C5Me5)2Os-H+‚‚‚OdPPh3, i.e. an Os-
H‚‚‚O hydrogen bond. Their conclusion was based upon
solution IR data which indicate diminished ν(Os-H)
and ν(PdO) stretching frequencies. This assertion is
supported in recent studies by Peris and Crabtree of a
number of cationic iridium hydrides that show the same
IR behavior indicative of LnIr-H+‚‚‚OdPPh3 hydrogen
bond formation.10 Pickett and co-workers11 have also
demonstrated the existence of M-H‚‚‚O hydrogen
bonds by obtaining crystallographic and NMR evidence
for a W-H‚‚‚O hydrogen bond in the compound [WH3-
(η1-OCOMe)(dppe)2] (dppe ) 1,2-bis(diphenylphosphi-
noethane)).
2020 (w), 1935 (w), 1898 (s) cm-1. Anal. Calcd. for C10H13
-
CoN2O4: C, 42.27; H, 4.61; N, 9.86. Found C, 39.85; H, 4.93;
N, 9.88. Better elemental analysis for this compound could
not be obtained.
-
(DABCO)H+Co(CO)3P P h 3 (2) was prepared by a modi-
fication of the procedure for 1. HCo(CO)4 prepared in situ was
first vacuum transferred to a toluene solution containing ca.
1 equiv of the phosphine ligand and stirred for 1 h prior to
introduction of the diamine. Typical byproducts in this
reaction include [HCo(CO)2(PPh3)2]12 and [Co2(CO)6(PPh3)2].
Compound 2 was originally isolated as a yellow crystalline
product by direct crystallization at -23 °C from an acetonitrile
solution of the crude mixture of products. Subsequently,
efforts to minimize byproducts included the use of less than 1
equiv of PPh3 (to prevent HCo(CO)2(PPh3)2 formation) and
reducing the amount DABCO present in the toluene solution
into which the initial HCo(CO)3PPh3 product is transferred.
Recrystallization from acetonitrile was then used to purify
material subsequently used in the NMR experiments. 1H
NMR (500 MHz, C6D5CD3, 298 K) δ 2.42 (s, CH2), δ 7.55 (t,
C6H5); (500 MHz, C6D5CD3, 183 K) δ 2.24, 1.66 (s, CH2), δ 8.06
(t, C6H5), 13.1 (s, NH). 13C NMR (125.5 MHz, C6D5CD3, 298
K) δ 46.9 (s, CH2), no signal observed for CO ligands; (125.5
MHz, C6D5CD3, 188 K) δ 43.4, 43.3 (s, CH2), no signal observed
for CO ligands. 31P NMR (202.5 MHz, C6D5CD3, 223 K) δ 63.7
ppm (s, PPh3) weak signal presumably due to quadrupole
broadening of 59Co; no signal observed at higher temperatures.
IR (CH3CN) ν CO 1928 (w), 1839 (s) cm-1; IR (C6H5Me) ν CO
1955 (w), 1857 (s) cm-1 (often additional bands at 2050 (w)
and 1972 (s), probably due to the presence of HCo(CO)3PPh3,
were also observed). Anal. Calcd for C28H28CoN2O3P: C,
62.55; H, 5.44; N, 5.40. Found C, 61.79; H, 5.66; N, 5.25.
X-r a y Cr ysta l Str u ctu r e Deter m in a tion s of 1 a n d 2.
Both crystal structures were solved by direct methods and
refined to convergence using the SHELXTL suite of pro-
grams.13 Data were corrected for absorption by semiempirical
methods.13 For 1 all non-hydrogen atoms were refined aniso-
tropically, ammonium hydrogens H(11) and H(21) were refined
isotropically, and all methylene hydrogens were refined using
a riding model with fixed isotropic displacement parameters.
For 2 the DABCO moiety exhibited orientational disorder
about the N‚‚‚N axis (staggered with respect to the carbonyl
groups, 67%; eclipsed with respect to the carbonyl groups,
33%). Non-hydrogen atoms were refined anisotropically,
except for carbon atoms of the minor orientation of DABCO.
Ammonium hydrogen H(1) was refined isotropically, and all
phenyl hydrogens were refined using a riding model with fixed
isotropic displacement parameters. The contributions of the
disordered methylene hydrogens were not included in the
model. Experimental data pertinent to both structure deter-
minations is given in Table 1.
We have previously described salts of the form
-
R3NH+Co(CO)4 that exhibit N-H‚‚‚Co hydrogen
bonds1b,c and sought to examine the similarities, and
differences, between such hydrogen bonds and (conven-
tional) hydrogen bonds which do not involve metal
atoms. In furthering these studies we have investigated
the effect of changing the basicity of the hydrogen bond
acceptor, Co, in salts of the type R3NH+Co(CO)3L- (L
) CO, PR3) by changing its ligand environment. Struc-
tural and spectroscopic studies of the salts (DABCO)-
H+Co(CO)3L- (DABCO ) 1,4-diazabicyclooctane; L )
CO, PPh3) were initiated. These studies are presented
together with ab initio calculations on the model com-
plexes Me3NH+Co(CO)3L- (L ) CO, PH3) and permit
the examination of the effect of increasing the basicity
at the metal center by replacing a CO ligand by a
phosphine ligand.
Exp er im en ta l Section
Gen er a l P r oced u r es. All manipulations of oxygen- or
water-sensitive materials were carried out under an atmo-
sphere of argon either using Schlenk-line techniques or in a
Vacuum Atmospheres drybox. NMR spectra were recorded on
either a Varian XL-300 or Bruker ARX-500 spectrometer. IR
spectra were recorded on a Perkin-Elmer 1600 Series FT-IR
spectrometer. Elemental analyses were carried out by Schwarz-
kopf Microanalytical Laboratory, Woodside, NY.
Resu lts a n d Discu ssion
(DABCO)H+Co(CO)4- (1) was prepared by methods simi-
The structures of 1 and 2 have been determined by
low temperature X-ray diffraction and are depicted in
Figure 1. Selected interatomic distances and angles are
-
lar to those described for other R3NH+Co(CO)4 salts.1b,c,6
HCo(CO)4 was prepared in situ and added by vacuum transfer
directly to a toluene solution of DABCO to give 1 as a colorless
powder, which was recrystallized under argon from THF
solution at 20 °C to yield crystals suitable for X-ray diffraction.
Maximum yield obtained, 79%. 1H NMR (300 MHz, C6D5CD3,
298 K) δ 2.18 (s, CH2); (300 MHz, C6D5CD3, 193 K) δ 1.74 (s,
listed in Table 2. 1 and 2 have structures similar to
- 1b,6
that reported for Et3NH+Co(CO)4
,
with an ap-
proximately linear N-H‚‚‚Co hydrogen bond (N-H‚‚‚
Co 173(3)° for 1; N-H‚‚‚Co 175(10)° for 2). The DABCO
methylene groups in 1 are staggered with respect to the
carbonyl groups. In compound 2 the phenyl groups of
(9) Epstein, L. M.; Shubina, E. S.; Krylov, A. N.; Kreindlin, A. Z.;
Rybinskaya, M. I. J . Organomet. Chem. 1993, 447, 277.
(10) Peris, E.; Crabtree, R. H. J . Chem. Soc., Chem. Commun. 1995,
2179.
(11) Fairhurst, S. A.; Henderson, R. A.; Hughes, D. L.; Ibrahim, S.
K.; Pickett, C. J . J . Chem. Soc., Chem. Commun. 1995, 1569.
(12) Zhao, D. Brammer, L. Inorg. Chem. 1994, 33, 5897.
(13) Sheldrick, G. SHELXTL 4.2, Siemens Analytical X-ray Instru-
ments, Inc., Madison, WI, 1991.

Strengthening of N-H‚‚‚Co Hydrogen Bonds
Organometallics, Vol. 15, No. 5, 1996 1443
Ta ble 1. Da ta Collection , Str u ctu r e Solu tion , a n d
Refin em en t P a r a m eter s for 1 a n d 2
Ta ble 2. Selected In ter a tom ic Dista n ces (Å) a n d
An gles (d eg) for 1 a n d 2
1a
2
1
2
Co(1)-C(101)
Co(1)-C(102)
Co(1)-C(103)
Co(1)-C(104)
C(101)-O(101)
C(102)-O(102)
C(103)-O(103)
C(104)-O(104)
1.777(4), 1.781(3) 2.172(2) Co-P
1.760(3), 1.760(3) 1.752(7) Co-C(1)
1.763(3), 1.771(3) 1.751(7) Co-C(2)
1.781(3), 1.769(3) 1.744(6) Co-C(3)
crystal color, habit
crystal size (mm)
colorless prisms yellow prisms
0.50 × 0.30 ×
0.50 × 0.25 ×
0.28
0.20
crystal system
space group, Z
a (Å)
b (Å)
c (Å)
monoclinic
P21/n, Z ) 8
8.728(2)
23.333(5)
12.146(2)
95.74(2)
2461.1(9)
1.534
orthorhombic
Pca21,a Z ) 4
16.084(8)
8.874(3)
17.312(3)
-
1.149(5), 1.146(4)
-
-
1.151(4), 1.156(4) 1.155(9) C(1)-O(1)
1.151(4), 1.155(4) 1.170(9) C(2)-O(2)
1.144(4), 1.148(4) 1.159(9) C(3)-O(3)
1.827(6) P-C(11)
â (deg)
1.829(6) P-C(21)
V (Å3)
2471(1)
1.394
1.824(6) P-C(31)
density (g/cm-3
)
N(11)-H(11)14
0.91(4), 0.97(4)
0.83(8)
N(1)-H(1)14
temperature (K)
135(5)
123(5)
X-ray wavelengthb (Å)
0.71073
1.398
0.71073
0.791
C(101)-Co(1)-C(102) 109.0(2), 106.5(1) 103.0(2) P-Co-C(1)
C(101)-Co(1)-C(103) 106.8(2), 107.6(1) 98.8(2) P-Co-C(2)
C(101)-Co(1)-C(104) 107.4(1), 104.9(1) 105.2(2) P-Co-C(3)
C(102)-Co(1)-C(103) 112.0(2), 111.2(2) 118.3(3) C(1)-Co-C(2)
C(102)-Co(1)-C(104) 107.6(1), 110.5(1) 113.8(3) C(1)-Co-C(3)
C(103)-Co(1)-C(104) 113.9(1), 115.4(1) 114.6(3) C(2)-Co-C(3)
µ
(Mo KR) (mm-1
)
2θ range (deg)
4.0-55.0
6195
4.0-55.0
5039
reflections collected
independent reflections (Rint
observed (F > 3.0 σ(F)) (n)
L.S. parameter (p)
R(F),c Rw(F)c
)
5580 (0.025)
4269
4636
3257
315
335
a
Values for both independent “molecules” are listed.
0.043, 0.043
1.21
0.065, 0.060
1.16
S(F)c
bond is strengthened upon traversing this series of
compounds, there is also an accompanying structural
trend associated with geometry about the metal center;
the equatorial carbonyl groups bend back away from the
N-H group to yield geometries closer to that adopted
by HCo(CO)3L (L ) CO, PR3) (Table 3), the product of
proton transfer to the metal. Thus, taken in sequence,
this series of structures serves as a model for progres-
sion along the pathway of electrophilic addition at a
tetrahedral center to yield a trigonal bipyramidal
product.
a
Absolute structure: Rodgers η-parameter ) 0.82(13). b Graph-
ite monochromated. c R(F) ) ∑|Fo - Fc|/∑Fo; Rw(F) ) [∑w|Fo
-
2
2
Fc| /∑wFo
]
1/2; S(F) ) [∑w|Fo - Fc|/(n - p)]1/2
.
In Table 4, IR stretching frequencies for the CO
ligands of 1 and 2 are compared with those for related
salts and for the corresponding cobalt hydride species.
Clear trends emerge. As anticipated, substitution of CO
ligands by P(OPh)3 and PPh3 ligands decreases the
stretching frequencies for the remaining CO ligands.
This conclusion, based primarily on the IR data of
Norton and co-workers,17 is reached both the for cobalt
hydride species and for the salts when measured in
acetonitrile solution. In such solutions, the latter are
present as solvent-separated ions, as is apparent from
F igu r e 1. (a) Structure of one independent “molecule” of
1. (b) Structure of 2, shown with the major (staggered)
orientation of the diamine unit. Each structure is shown
with 50% probability ellipsoids for non-hydrogen atoms.
-
the data for R3NH+Co(CO)4 salts, which exhibit only
the T2 ν(CO) band characteristic of an anion of Td
symmetry. However, in toluene (or indeed benzene)
solution the corresponding IR data are consistent with
the salts adopting the associated form present in the
solid state, i.e. in which N-H‚‚‚Co hydrogen bonding is
present. This is particularly evident in the R3N-
the phosphine are staggered with respect to the carbonyl
ligands, but disorder of the DABCO moiety about the
approximate molecular 3-fold axis results in the pres-
ence of both staggered and eclipsed conformations of the
methylene groups in a 67:33 ratio. The key structural
parameter from the X-ray diffraction studies is the N‚‚‚
Co separation. This separation is substantially reduced
-
H+Co(CO)4 salts which now exhibit three bands (2A1
+ E) consistent with C3v symmetry, the new bands
appearing at higher frequencies. Such a change in
symmetry cannot occur for 2. However, it is noted that
-
in 1 (3.437(3) Å) relative to Et3NH+Co(CO)4 (3.684(2)
Å at 123 K)1e and is further reduced in 2 (3.294(6) Å)
relative to 1. Thus, replacement of Et3N by the less
sterically demanding base DABCO results in a strength-
ening of the N-H‚‚‚Co hydrogen bond. Furthermore,
increasing the basicity of the metal center also strength-
ens the hydrogen bond. Necessarily the bridging hy-
drogen more closely approaches the metal center,14 and
we presume that a concomitant lengthening of the N-H
bond should take place. As the N-H‚‚‚Co hydrogen
(15) (a) McNeill, E. A.; Scholer, F. R. J . Am. Chem. Soc. 1977, 99,
6243. (b) Leigh, J . S.; Whitmire, K. H. Acta Crystallogr. 1989, C45,
210.
(16) Preliminary NMR and crystallographic data for the salt
(DABCO)H+Co(CO)3PPh2(p-tol)- also support the observations made
for 2. Variable-temperature NMR data for (DABCO)H+Co(CO)3PPh2-
(p-tol)- suggest that the coalescence point for the fluxional process lies
in the range 215-240 K, but the exact temperature has not yet been
resolved due to overlapping peaks in this region of the spectrum.
Preliminary analysis of single crystal X-ray diffraction data from a
weakly diffracting crystal of (DABCO)H+Co(CO)3PPh2(p-tol)- as its
acetonitrile solvate confirms that the connectivity of this salt is
analogous to that of 2. However, disorder of both the DABCO and PPh2-
(p-tol) units have prevented an accurate analysis.
(14) Assuming a N-H separation of 1.05 Å, as determined by
- 1b
neutron diffraction for Et3NH+Co(CO)4
1
,
the H‚‚‚Co separations in
and 2 are 2.39 Å (mean) and 2.25 Å, respectively. Observed
separations based on refined hydrogen positions are necessarily longer
at 2.48(4) and 2.52(4) Å for 1 and 2.47(7) Å for 2.
(17) Moore, E. J .; Sullivan, J . M.; Norton, J . R. J . Am. Chem. Soc.
1986, 108, 2257.

1444 Organometallics, Vol. 15, No. 5, 1996
Zhao et al.
Ta ble 3. Com p a r ison of th e Geom etr ies of Sa lts w ith N-H‚‚‚Co Hyd r ogen Bon d s w ith Th a t of HCo(CO)3L
(L ) CO, P R3)
N‚‚‚Co,
Ceq-Co-Ceq
,
Lax-Co-Ceq
,
temp
(K)
compound
Å
deg
deg
X/N/Ea
ref
Et3NH+Co(CO)4
3.684(2)
3.639(1)
3.437(3)
3.402(4)
3.294(6)
na
112.4(1)
112.1(9)
111.2(2)
113(1)
115(3)
117.3
106.3(1)
106.7(7)
107.2(2)
106(1)
103(3)
99.7(6)
99(2)
123.0(5)
135(5)
135(5)
295
123(5)
238
N
1e
1c
d
6
d
-
Xc
Xc
Xc
Xc
E
b
[(NMP)3H2][Co(CO)4]2
(DABCO)H+Co(CO)4- (1)
Me3NH+Co(CO)4
-
(DABCO)H+Co(CO)3PPh3- (2)
HCo(CO)4
15a
15b
HCo(CO)3PCy3
na
118(2)
296
Xc
e
a
b
X ) X-ray diffraction, N ) neutron diffraction, E ) electron diffraction. NMP ) N-methylpiperazine. c Averaged dimensions, with
esds (in parentheses) calculated according to [∑(di - d)2/n(n - 1)]1/2 which thus reflect the scatter in the (three) observed values, rather
than esds of the individual values. This work. e Cy ) cyclohexyl.
d
Ta ble 4. In fr a r ed Ca r bon yl Str etch in g F r equ en cies (cm -1) of HCo(CO)3L a n d A+Co(CO)3L- Com p ou n d s (L
) CO, P (OP h )3, P P h 3; A+ ) Na +, R3NH+)
anion
symmetry
symmetry
of bands
hydrogen
bond
compound
ν(CO), cm-1
solvent
17
HCo(CO)4
2117, 2054, 2023
2075, 1996
2050, 1971
2015, 1934, 1895
2015, 1931, 1895
1892
2020, 1935, 1898
1892
1961, 1872
1955, 1857
1928, 1839
1927, 1838
CH3CN
CH3CN
CH3CN
C6H5Me
C6H5Me
CH3CN
C6H5Me
CH3CN
CH3CN
C6H5Me
CH3CN
CH3CN
C3v
C3v
C3v
C3v
C3v
Td
C3v
Td
C3v
C3v
C3v
C3v
2A1 + E
A1 + E
A1 + E
2A1 + E
2A1 + E
T2
2A1 + E
T2
A1 + E
A1 + E
A1 + E
A1 + E
na
na
na
17
HCo(CO)3P(OPh)3
17
HCo(CO)3PPh3
Me3NH+Co(CO)4
N-H‚‚‚Co
N-H‚‚‚Co
none
N-H‚‚‚Co
none
none
N-H‚‚‚Co
none
-
Et3NH+Co(CO)4
-
Et3NH+Co(CO)4
-
(DABCO)H+Co(CO)4- (1)
(DABCO)H+Co(CO)4- (1)
Na+Co(CO)3P(OPh)3
- 17
(DABCO)H+Co(CO)3(PPh3)- (2)
(DABCO)H+Co(CO)3(PPh3)- (2)
Et3NH+Co(CO)3(PPh3)- 17
none
Ta ble 5. Ch em ica l Sh ifts (p p m ) of th e Meth ylen e
Gr ou p s of DABCO for 1 a n d 2 in Tolu en e-d 8
Solu tion
On the basis of the observed crystal structures, the
IR data (Table 4), and the fact that a N-H signal but
1
no Co-H signal is observed in the H NMR spectrum,
temp 1H NMR 13C NMR
1H NMR
13C NMR
1H NMR
we assume that, in toluene solution, the N-H‚‚‚Co
hydrogen bonded species (B, vide infra) is the resting
state of the fluxional system. We therefore infer from
the temperature-dependent NMR data that a process
which interconverts the methylene protons of DABCO
by separation of the DABCO-containing species from the
cobalt-containing species occurs, and that this process
is retarded when PPh3 is substituted for CO. The
shortening of the N‚‚‚Co separation indicated by the
crystallographic studies suggest a possible explanation,
namely that an increased N-H‚‚‚Co hydrogen bond
strength in the phosphine-substituted system decreases
the tendency of the zwitterionic complex to separate into
the component ions. While we will suggest that this is
indeed the case, such an effect cannot be inferred from
the NMR data alone as there remains some ambiguity
over the pathway for the fluxional process. We will not
focus here on the intra- versus intermolecular nature
of the process; the key issue is that the fluxional process
could conceivably involve one of two pathways for
dissociation/reassociation (Scheme 1).
(K)
for 1
for 1
for 2
for 2
for DABCO
298
253
233
213
193
188
2.18
-
2.05
1.87
1.74
-
47.2
-
2.42
2.16
46.9
-
2.40
2.40
2.40
2.40
2.40
-
-
2.22, 1.84
2.26, 1.70
2.26, 1.67
44.2a
-
-
-
-
43.9
2.24,b 1.66b 43.4, 43.3
a
b
At 235 K. At 183 K.
an increase in the carbonyl stretching frequencies does
occur upon formation of the N-H‚‚‚Co hydrogen bond,
i.e. on going from acetonitrile solution to toluene solu-
tion. Such a change in the frequencies of the ν(CO)
bands is consistent with the role of the metal center as
a Lewis base in forming the N-H‚‚‚Co hydrogen bond.
1H NMR spectra for 1 and 2 in toluene-d8 solution at
298 K show only a singlet for the methylene hydrogens
of the DABCO. The N-H proton is not observed. Upon
lowering the temperature the methylene signal moves
upfield and the N-H signal is observed at low field (1
δ 10.1, 2 δ 13.1). In addition, for 2 the methylene signal
broadens and separates into two signals at tempera-
tures below ca. 235 K.16 The methylene signal for 1
remains a singlet down to a temperature of 193 K (Table
The simplest process (I) mentioned above is the
cleavage of the N-H‚‚‚Co hydrogen bond in the sug-
gested resting state (B) to yield the two ions (A).
Alternatively, transfer of a proton (II) could yield
another hydrogen-bonded complex (C) incorporating
neutral DABCO and cobalt-hydride moieties. Cleavage
of this hydrogen bond (III) would allow the interchange
of the DABCO coordination site via free DABCO (D).
1
5). Other H NMR data show no apparent temperature
dependence. These data suggest a fluxional process in
which the hydrogen bond is broken and the diamine can
either undergo “intermolecular” interchange with other
diamines prior to reformation of the hydrogen-bonded
salt, or reform the hydrogen bond using the nitrogen
atom at the opposite end of the same molecule, i.e.
“intramolecular” interchange. This fluxional process
necessarily involves proton transfer between nitrogen
sites.
To investigate the expected effects of PPh3 substitu-
tion on the second pathway we have performed a series
of ab initio SCF calculations on the model complexes

Strengthening of N-H‚‚‚Co Hydrogen Bonds
Organometallics, Vol. 15, No. 5, 1996 1445
Sch em e 1.
P ossible Dissocia tion /Rea ssocia tion P a th s18 for
(DABCO)H+‚‚‚Co(CO)3L- (L ) CO, P P h 3)
F igu r e 2. Plot of relative energy vs N-H separation from
ab initio SCF calculations. Curve A is for Me3NH+‚‚‚Co(CO)4
-
and curve B for Me3NH+‚‚‚Co(CO)3PH3
.
(Me3NH+‚‚‚Co(CO)3L-, L ) CO and PH3).19 We adopted
the same skeletal geometry for the two complexes, and
computed the energy profile as the proton migrates from
N to Co, shown in Figure 2. In both cases, the potential
energy surface shows two distinct minima corresponding
to species of type (B) (N-H ca 1.05 Å) and (C) (N-H
ca. 1.80 Å). From the energy curves it is clear that the
calculations predict that the most stable conformation
considered is the cobalt hydride (C in Scheme 1). The
discrepancy between this result and that observed
experimentally (B) can be attributed to the fact that the
computations were performed in vacuo, where there is
no dielectric medium to stabilize the separation of
charge characteristic of the zwitterionic system (B).
We might anticipate that if the species were soluble
in nonpolar media the resting state would be of type
(C), and pathway III would be the preferred dissociation
route.
substitution. As expected from considerations of the
charge density at Co the stability of the cobalt hydride
complex with respect to the zwitterionic species is found
to increase by 6 kcal/mol on replacement of CO by PH3.
Clearly this also has the effect of lowering the barrier
to the conversion B f C. Turning now to step III, we
would anticipate that phosphine substitution at Co
would increase the charge density at the hydride ligand
and thereby weaken its potential as a hydrogen bond
donor. Therefore, both processes II and III would be
expected to become more facile on phosphine substitu-
tion. Taken together with the NMR evidence that the
observed process shows the opposite behavior, we infer
that our original supposition of an ionic pathway (I) is
correct. The NMR evidence thus provides support for
the crystallographic evidence presented herein, that the
N-H‚‚‚Co hydrogen-bonding interaction is strengthened
when the basicity of the metal-containing fragment is
increased.
Of more relevance to the current discussion is the
change in the energy profile produced by phosphine
(18) Only illustration of the possible “intramolecular” processes are
shown in the scheme. From the present data, it is not possible to
distinguish between these and related “intermolecular” processes which
may occur.
Ack n ow led gm en t. L.B. is grateful for financial
support from the Donors of the Petroleum Research
Fund, administered by the American Chemical Society,
and from the University of Missouri Research Board.
L.B. and P.S. thank NATO for a Collaborative Research
Grant (CRG-920164). The Bruker ARX-500 NMR spec-
trometer was purchased with support from the U.S.
Department of Energy, Grant No. DE-FG02-92CH10499.
Support for the Varian XL-300 NMR spectrometer from
NSF grant CHE-8506671 is also acknowledged.
(19) (a) All calculations were with the GAMESS-UK19b package.
DZ quality basis sets were used for all atoms19c,d and augmented with
polarization functions on the heavy atoms of the NMe3 group (C d
exponent 0.75, N d exponent 0.80) and the hydrogen-bonded H (p
exponent 1.0). Additional diffuse p (exponent 0.0525) and d (0.0783)
functions were added to the Co basis. The model heavy-atom geom-
etries were based on the X-ray results with Co‚‚‚N set to 3.36 Å. (b)
GAMESS-UK is a package of ab initio programs written by M. F.
Guest, J . H. van Lenthe, J . Kendrick, K. Schoffel, P. Sherwood, and
R. J . Harrison, with contributions from R. D. Amos, R. J . Buenker, M.
Dupuis, N. C. Handy, I. H. Hillier, P. J . Knowles, V. Bonacic-Koutecky,
W. von Niessen, V. R. Saunders, and A. J . Stone. The package is
derived from the original GAMESS code due to M. Dupuis, D. Spangler,
and J . Wendoloski, NRCC Software Catalog, Vol. 1, Program No. QG01
(GAMESS), 1980. (c) For C, H, O, N: Dunning, T. H. J . Chem. Phys.
1970, 53, 2823. (d) For Co, P: Huzinaga, S.; Andzelm, J .; Klobukowski,
M.; Radzio-Andzelm, E.; Sakai, Y.; Tatewaki, H. Gaussian Basis Sets
for Molecular Calculations; Elsevier: Amsterdam, 1984. The basis was
contracted to (8s,6p,2d) as suggested by Dunning and Hay (Dunning,
T. H., J r.; Hay, P. J . Methods of Electronic Structure Theory; Shaefer,
H. F., III, Ed.; Plenum Press: 1977; p 1.)
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data, positional and displacement parameters, in-
teratomic distances and angles for 1 and 2; figures showing
the other independent “molecule” of 1 and disorder model for
2 (18 pages). Ordering information is given on any current
masthead page
OM950777E