Organometallics 1995, 14, 3327-3334
3327
Does the Tetrahydroborate Species AuB& Exist? Ab Initio MO Study of the Structure and Stability of CUB&, AgB&, and AuB& Djamaladdin G. Musaev and Keiji Morokuma* Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322 Received February 27, 1995@ The MP2, MP4(SDTQ), QCISD(T), and CCSD(T) methods in conjunction with the 19-valence electron Hay-Wadt relativistic effective core potential have been used t o study geometries, energies, and possible rearrangements of tetrahydroborate compounds of Cu, Ag, and Au. For the CuBH4 and AgB& molecules only the tetrahydroborate structure MB& can exist; the hydrido borane structure, HMBH3, is thermodynamically (about 25-26 k c d mol) and kinetically unstable and rearranges into the MBH4 structure. The bidentate (b) and tridentate (t)MBH4 structures are energetically very close and structurally nonrigid, and at ambient temperature their bridge and terminal hydrogen atoms will easily interchange by the pathway (b) t (t) t (b') t (t') t .... However, the Au analog cannot exist as a tetrahydroborate species, AuBH4, which rearranges with a small (-1 k c d m o l ) barrier to the hydrido(borane)gold,HAuBH3, structure lying 11.7 kcaYmol lower. The thermal stability of MBH4 iM = Cu, Ag, and Au) molecules for the lowest decomposition path, MH +BH3, decreases in the order Cu > Ag > Au.
Introduction The tetrahydroborate ion, BH4-, is the simplest known anionic boron hydride, which is known t o coordinate to a metal ion via a monodentate (only in one known case), bidentate, or tridentate manner:l-12
properties have been studied.'-12 In general, the bridge Hband terminal Ht hydrogen atoms of these complexes interchange at ambient temperatures. The actual rate of the process and the mechanism of rearrangement, however, has remained to be solved. Various possible pathways have been proposed. The first pathway suggested to explain stereochemical nonrigidity in a tetrahydroborate complexes involves permutation of bridge and terminal hydrogens via a monodentate intermediate or transition state:
bidcntate, (b)
monodentate, (m)
nidcntate, (t)
Tetrahydroborate complexes of various metals have been synthesized, and their chemical and physical @Abstractpublished in Advance ACS Abstracts, June 1, 1995. (11Muetterties, E. L. Boron Hydride Chemistry; Academic Press: New York, 1975. (2)Marks, T. J.; Kolb, J. R. Chem. Rev. 1977,77,263. (3)Housecroft. C. E.: Fehler, T. P. Adv. Orgunomet. Chem. 1981, 21,57. (4)Edelstein, N. Inorg. Chem. 1981,20,299. (5)Corazza, F.;Floriani, A. G.; Cheisi-Villa A,; Guastini, C. Inorg. Chem. 1991,30,145. (6)Csaszar, A.G.; Hedberg, L.; Hedberg, K.; Burns, R.; Wen, A. T.; Mcglinchey, M. J. Inorg. Chem. 1991, 30, 1371. (7)White, J. P.; Deng, H.; Shore, S. G. Inorg. Chem. 1991,30,2337. 18) Gozum, J. E.; Girolami, G. S. J.Am. Chem. Soc. 1991,113,3829. (9)Gozum, J. E.;Wilson, S. R.; Girolami, G. S. J.Am. Chem. SOC. 1992,114,9483. 110)Electron Deficient Boron and Carbon Chemistry; Olah, G. A., Wade, K., Williams, R. E., Eds.; Wiley Interscience: New York, 1990. 111)Kawashima, Y.; Yamada, C.; Hirota, E. J. Chem. Phys. 1991, 94,7707. (12)Kawashima, Y.; Hirota, E. J. Chem. Phys. 1992,96,2460.
Certain empirical observations, as well as ab initio calculations13-30have suggested that, at least for early (13)Boldyrev, A. I.; Charkin, 0. P.; Rambidi, N. G.; Avdeev, V. I. Chem. Phys. Lett. 1976,44,20. (14)Baranov, L. Ya.; Boldyrev, A. I. Chem. Phys. Lett. 1983,96, 218. (15)Kello, V.; Urban, M.; Boldyrev, A. I. Chem. Phys. Lett. 1984, 106,455. (16)Bonaccorsi, R.; Scrocco, E.; Tomasi, J. Theor. Chim. Acta 1979, 52,113. ( 17)Dill, J. B.; Schleyer, P. v. R.; Binkley, J. S.; Pople, J. A. J.Am. Chem. SOC.1977,79,6159. ( 18)Barone, V.; Dolcetti, G.; Lelj, F.; Russo, N. Inorg. Chem. 1981. 20. ~ 1687. -., . . (19)Charkin, 0.P.; Musaev, D. G.; Klimenko, N. M. Russ. J. Coord. Chem. 1985. - .. . ----,1 1 . 728. -(20)DeFrees, D. J.; Raghavachari, K.; Schlegel, H. B.; Pople, J. A,; Schleyer, P. v. R. J.Phys. Chem. 1987,91,1857. 1211 Musaev, D. G.;Zyubin, A. S.; Charkin, 0. P. Russ. J. Coord. Chem. 1988,14,638.
--.
0 1995 American Chemical Society
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Organometallics, Vol. 14, No. 7, 1995
Musaev a n d Morokuma
transition metals, bidentate and tridentate structures do not differ much in energy. The second suggested pathway is a concerted process for tetrahydroborates having bidentate reactant geometries to twist the BH4 ligand about one of the B-Hb bonds:
the phosphine, arsine, or stibine ligand.37.38However, their gold analog has not been prepared even at low temperature. The Au(BH413 species prepared at -120 "C in diethyl ether by reaction of gold(II1)chloride with LiBH4 is thermally very unstable and decomposes to gold metal, diborane, and hydrogen upon increasing the t e m p e r a t ~ r e .Thus, ~ ~ it is apparent that the stability of the MBH4 complexes decreases in the order of Cu > Ag =. Au, and we believe that the quantum chemical calculations carried out in this paper for study of structure, stability, and possible mechanisms of intermolecular process of the CuBH4, AgBH4, and AuBH4 species will be helpful for elucidation of this stability difference.
A similar twisting process can be written for a complex with a tridentate reactant structure; here the midpoint of the reaction coordinate resembles, with the exception of possible asymmetry in the bridge, a bidentate configuration. For the interchange process of bridge and terminal hydrogens, a Berry pseudorotation mechanism has also been suggested.2 The scope of dynamic molecular processes involving the coordinated tetrahydroborate moiety is not limited to intramolecular bridge-terminal hydrogen interchange. Hydrogen exchange between tetrahydroborate moiety and other ligands coordinated to transition metal atoms also may occur. This process has been widely studied in the literature. For example, variable-temperature lH NMR spectra of ($-CsH5)2Zr(BH4)2, ($C5H5)zHffBH4)2, and ($-C5H5)2Zr(H)B& exhibit a rapid interchange of C5H5 and BH4 hydrogens at high temp e r a t ~ r e .In ~ ~the present paper we will concentrate on the hydrogen exchange between the tetrahydroborate moiety and transition metal center, MBH4 * HMBH3 and study some possible mechanisms of the intermolecular exchange process for CuBH4, AgB&, and AuB& species. The observed differences between the stability of the CuBH4, AgBH4, and AuBH4 species also need t o be elucidated. The CuBH4 ~ p e c i e s ~and ~ - the ~ ~ AgBH4 analog6 have been synthesized at low temperature in diethyl ether and have been shown to be air sensitive and decompose to metal atoms, diborane, and hydrogen a t -12 and -30 "C, respectively. These compounds can be stabilized by the addition of soft Lewis bases to form complexes of the type (R3EhMBH4, where R3E may be
Calculation Procedure
(22)Charkin, 0. P.; Bonaccorsi, R.; Tomasi, J.; Zyubin, A. S.; Musaev, D. G. Russ. J. Inorg. Chem. 1987,32, 2644. (23)Charkin, 0. P.; Bonaccorsi, R.; Tomasi, J.; Zyubin, A. S.; Musaev, D. G. Russ. J. Inorg. Chem. 1987,32, 2907. (24)Musaev, D. G.; Charkin, 0. P. Russ. J. Inorg. Chem. 1991,36, 430. (25)Ramondo, F.; Bencivenni, L.; Di Martino, V. Chem. Phys. 1991, 158, 41. (26)Lledos, A.; Duran, M.; Jean, Y.; Volatron, F. Inorg. Chem. 1991, 23., 4440. ~. ~~~(27)Lledos, A.; Duran, M.; Jean, Y.; Volatron, F. Bull. SOC.Chim. Fr. 1992, 129, 216. 128) Volatron, F.; Duran, M.; Lledos, A,; Jean, Y. Inorg. Chem. 1993, .?2. - -, 9.51. - - -. (29)Jarid, A.; Lledos, A.; Jean, Y.; Volatron, F. Inorg. Chem. 1993, 32, 4695. (30)Francisco, J. S.; Williams, I. H. J . Phys. Chem. 1992,96, 7567. (31)Marks,T. J.; Kolb, J. R. J. Am. Chem. SOC.1975, 97, 3397. (32)Wiberg, E. Angew. Chem. 1953, 65, 16. (33)Klingen, T. J. Inorg. Chem. 1964, 3, 1058. (34)Klingen, T. J. J . Inorg. Nucl. Chem. 1966,28, 2243. (35)Wiberg, E. Henle, W. 2.Nuturforsch., B 1962, 7, 582. (36)Wiberg, E. Henle, W. Z. Nuturforsch., B 1952, 7, 575.
All calculations have been performed by using the GAUSSIAN-92 program.40 Geometries have been optimized by using the MP2 method in conjunction with the lanl2dz (for transition metal atoms) and the 6 - 3 1 G ( d , ~ )(for ~ l other atoms) basis sets, which will be called basis set I. The lanl2dz basis set taken from the GAUSSIAN-92 library includes Hay-Wadt effective core potential42 in which the 19 nsnpndh 1)s electrons of the group 11 metals are explicitly considered, with the following basis sets of (8s5p5d/3s3p2d), (8s6p4d/3s3p2d), and (8s6p3&3s3p2d)for Cu, Ag, and Au, respectively. Note that the present ECP takes into account the relativistic effects only for second- and third-row transition metal atoms. Due to the absence of the relativistic effects our results probably will underestimate the bonding energies by a few kcaVmol for the Cu complexes. However, it will not affect the calculated relative energies. Once the geometries were determined, energies have been recalculated by using QCISD(T), MP4(SDTQ), and CCSD(T1methods with the lanl2dz plus polarization f-functions for transition metal atoms and 6-311G(d,p) for other atoms, called basis set 11. The exponents of the polarization f-functions have been taken from the recent paper of Ehlers et al.43 Vibrational frequencies have been numerically calculated at the MPW level for characterization of the nature of the stationary points, zero-point correction (ZPC), and prediction of vibrational spectra. All the stationary points have been positively identified for minima (number of imaginary frequencies, NIMAG = O),transition state (NIMAG = 1) and higher order stationary points. In some important cases, we have followed the "pseudo-intrinsic reaction coordinate (IRC)" from a given transition state to both directions to confirm the reactant and the product to which this transition state lead; we took a few steps of the IRC44from the transition state and then performed geometry optimization toward a local minimum. The final energy parameters given in the paper
+
(37)Lippard, S. J.; Ucko, D. A. Inorg. Chem. 1968, 7, 1051. (38)Cariati, F.; Naldini, L. Guzz. Chim. Itul. 1965, 95, 201. (39)Puddephatt, R. J. The Chemistry of Gold; Elsevier: Amsterdam, 1978. (40)GAUSSIAN-92: Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A,; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzales, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J . ; Stewart, J . J. A,; Pople, J . A. Gaussian Inc., Pittsburgh, PA, 1992. (41)( a ) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973,28, 213. ( b ) Francl, M. M.; Pietro, W. J . ; Henre, W. J.; Binkley, J . S.; Gordon, M. S.; DeFrees, D. J.: Pople, J . A. J . Chem. Phys. 1982, 77, 3654. (42)Hay, P. J.; Wadt. W. R. J . Chem. Phys. 1986, 82, 299. (43)Ehlers, A. W.; Bohme, M.; Dapprich, S.; Gobbi, A.; Hollwarth, A.; Jonas, V.; Kohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993,208, 111. (44)( a )Ishida, K.; Morokuma, K.; Komornicki, A. J . Chem. Phys. 1977,66,2153.tb) Gonzales, C.; Schlegel, H. B. J . Phys. Chem. 1989, 90, 2154.
Organometallics, Vol. 14,No. 7, 1995 3329
CuBH4,AgBH4, and AuBH4 contain the ZPC calculated with the MPm normal frequencies scaled by 0.95.45 Results and Discussions
The MP2/I optimized geometries are shown in Figures 1 and 2. The relative energies of various structures of MBH4 calculated at several levels of theory are shown in Table 1. The calculated binding energies of M-H, M-BH3, H-MBH3, and HM-BH3 bonds are given in Table 2. Results of Mulliken population analysis for these molecules are given in Table 3. The relative energies of all the structures involved are summarized in Figure 3. Geometries of Intermediates and Transition States. Here we will consider geometries of all the critical points we find; their energies and nature (i.e., NIMAG) will be discussed in the next subsection. MP2A optimized geometrical parameters given in Figure 1 show that geometries of the BH4 group in the CW assumed bidentate (b) and Ca,-assumed tridentate (t) structures are weakly dependent on the nature of metal atoms; the distances r(B-Hb) and r(B-Ht) change only up to 0.007 and 0.02 A, respectively, among M = Cu, Ag, and Au, and are 0.01-0.05 8, longer than in the isolated BH4- anion, shown in Figure 2. On the other hand, the distance r(B-Hb) in the C3,-assumed monodentate (m) structures for M = Cu and Ag is about 0.1 A longer than in isolated BH4-. It is dramatically stretched for M = Au; the distance r(B-Hb) in AuBH4 (m) is about 0.40 and 0.50 A longer than that in the (m) structure of and in isolated B&-, respectively, with the bridged hydrogen Hb actually closer to Au (1.529 A) than to B (1.757 A). Upon optimization without any symmetry constraint the geometry of AuBH4 converged to a structure having C,, symmetry between the monodentate and tridentate structures, (mt) in Figure 1, where dB-Hb) is 1.797 A, about 0.57 A longer than that in isolated BH4-. One may say that in the AuBH4 (m) and (mt) structures the B-H bond is already strongly activated. The geometries of the HMBH3 complex, (h), are different among M = Cu, Ag, and Au, as seen in Figure 1. For Cu it has a planar C, structure, where two H atoms of BH3 group are asymmetrically oriented to the metal atom. The structure for Ag is similar to that for Cu; two H atoms of BH3 group are also asymmetrically oriented t o the metal atom, but the entire molecule is slightly nonplanar with the Ht'AgB angle of 169" and the Ht'AgBHb dihedral angle of 43". Both for Cu and Ag the geometry of the BH3 fragment is not much M e r e n t from that in isolated BH3 and the M-B distance is quite long, indicating that the interaction between MH and BH3 is not stronp. For Au, however, the structure is very different. It has a nonplanar C,, structure, with only one H atom of BH3 directed to the metal atom. The interaction of Au with the bridging hydrogen atom Hb seems t o be very stron , as indicated The Ht'MB by the B-Hb bond elongated by 0.10 angle decreases in the order of M: Cu (172")sz Ag (169") > Au (136"). These structural features will be discussed later in connection with the Mulliken population analysis.
R
(45) Hehre, W. J.;Radom, L.; Schleyer, P. v. R.;Pople, J. A. Ab initio Molecular Orbital Theory; Wiley; New York, 1986.
The transition states (ts) for insertion of the Cu and Ag metal center into a B-H bond of BH4 are located late, as seen in Figure 1, with their structures resembling more the structures (h) of the HMBH3 product than the lowest structures (t)of the MBH4 reactant. The Cu compound has C1 symmetry and is quasiplanar, where two H atoms of the BH3 fragment are oriented to the Cu atom. The Ag compound has C, symmetry, with one H atom of the BH3 fragment is oriented to the Ag atom. For Au the C,,transition state (ts) is tight with a short Au-H distance and resembles more the MBH4 reactant. Qualitative Characteristics of the Potential Energy Surface. Here we discuss the qualitative characteristics of the potential energy surface (PES)at the MP2A ZPC level of theory. More quantitative energetics for critical structures a t higher levels of theory will be discussed in the next subsection. For CuBH4 and AgBH4 the (t)structure is the global minimum. As shown in Figure 1, the (b) structure for CuBH4 and AgBH4 is only 0.58 and 0.24 kcal/mol, respectively, higher in energy than the (t) structure. The normal mode analysis shows that the (b) structure has one B1 normal mode with a small imaginary frequency shown in Figure 1 and is the transition state between one (t) and another (t) structure. Thus, like their alkali and alkaline earth a n a l o g ~ , l l CuBH4 - ~ ~ ~ ~and ~ AgBH4 are structurally nonrigid and at ambient temperatures should exchange bridge and terminal hydrogen atoms easily by the pathway (t)s (b)2 (t')2 (b') 2 .... With a smaller barrier, AgBH4 is a little more fluxial than CuBH4. The monodentate (m) structure lies about 21 and 13 kcal/mol higher than the (t)structure for M = Cu and Ag, respectively. The (m) structure has a doubly degenerate imaginary frequency connecting (t)and (b) structures. The hydridoboranemetal, HMBH3 (h), structure is a local minimum on the potential energy surface for CuBH4 and AgBH4. It lies 26.0 and 27.4 kcal/mol higher than the (t)structure, has a barrier of only 0.16 and 1.88 kcal/mol, respectively, at the transition state (ts) for rearrangement to the (t) structure, and most likely does not exist at moderate temperature. The transition state (ts) separating the HMBH3 (h) structure from the MBH4 (t)or (b) structure has one small imaginary frequency of 146i and 151i cm-l for CuBH4 and AgBH4, respectively, corresponding to migration of one of the hydrogen atoms (Ht')from B to the metal atom, as shown in Figure 1. For Cu, following this normal mode, the transition state can structurally easily reach the product (h) having a very similar structure. Pseudo-IRC following indicates that the path from the transition state back to the reactant leads to the reactant (t). For Ag, pseudo-IRC following shows that it can reach the product (h) by a 125" rotation of the BH3 group accompanied by a concurrent 63" conrotatory motion of Ht'. A large reversal rotational and translational motion of Ht' from (ts) accompanied with the rotation of terminal the Ht atoms maintaining the overall C, symmetry connects (ts) to the reactant (b), which can be converted t o the global minimum (t)via rotation of BH4. For AuBH4 the calculated potential energy picture is completely different from that for CuBH4 or AgBH4. Here the local minimum is the (mt) structure, and the structure (b), 1.7 kcal/mol higher with one Bz imaginary
+
3330
Organometallics, Vol. 14, No. 7, 1995 H'$
Musaev and Morokuma 56.40 1.285 (57.57) (1.272) [59.39] [1*2943a
I
.AH'
(m) C k (assumed)
457i cm-' (E) (379i) cm-' (E) [214i] cm-' (E)
21.37 kcal/mol (13.02) kcaYmol [3.73] kcal/mol
tHb
I /
2.100
' Ut
[ 1 1 8 . 4 6 p H' (b) Czv(assumed) 15i cm-' (B1) 0.58 kcal/mol (67i) cm-' (B1) (0.24) kcaVmol [334iI cm-' (B2) [1.70] kcavmol
(77*5\6)
(27.37) kcal/mol (Nimag = 0)
(h) C, 26.18 kcal/mol Nimag = 0
m
(1.183)
(HbBH') = (1 15.70) (H'AgBH') = (-1 16.80) (H' AgBHb) = (42.74) (H"AgBH') = (-147.80)
961 H'
[-7.101 kcaVmol dihedral angle: [Nimag = 01 (H'IAuBH') = [71.82]
8""
(ts)
c1
146i cm'' dihedral angles : 26.02 kcal/mol ( H"CuBH' ) = 63.61 ( H"CuBHb) = -143.96 ( H"CuBH') = -75.40
(ts)
c,
[82i] cm-1 (A') dihedral angle: i0.421 kcaVmol ( H"AuBH'")=[ 119.701 ( H"AUBH' )=[o.o]
Figure 1. MP24 optimized geometries (distances in A and angles in deg) and the MP24 + ZPC relative energies (in kcal/mol) of various structures of CuBH4, AgBH4 (in parentheses), and AuBH4 (in brackets).
Organometallics, Vol. 14,No. 7, 1995 3331
CuBHJ,AgBHJ, and AuBHJ
1.186
H' [ 119.90 ]
cs
(m)
7.60kcaVmol
Nimag=O
( 3.93 ) kcallmol (Nimagd) [ 4.23 ] kcaVmol [Nimag=O]
Bb
7.83 kcaVmo1
55i cm-'031)
( 3.99 ) kcaVmol
( 22i cm-' [ 52i cm'' (Bl)]
[ 4.37 1 k d h "
m1))
t
H'
Hb
p.221
(t)
c3v
H'
( 0.00 ) kcaVmol (Nimag=O) [ 0.00 3 kcaVmol [Nimag=O]
1.497
1.4626_+0.0003
Figure 2. MP2A optimized geometries (distances in A and angles in deg) and the MP2A + ZPC relative energies (in kcaumol) of various structures of the MBHS, MH, BHS, and BH4- molecules, where M = Cu, Ag (in parentheses), and Au (in brackets). Table 1. Relative Energies (in kcallmol) of Various Structures of Metal-Borane Complexes, CUB-, AgBH4, and AuBH4, at the MP2/I Optimized Geometriesu basis set I1
basis set I mo1ecu1e ( struct 1
MP2 20.41 0.55 -222.28951
25.81 25.99 12.93 0.20 - 172.05552 25.37 26.79 2.82 1.24 7.39 - 161.69518
-6.90 0.19
NIMAGb
2 1 0 0 1
4571' 115i
146i 379i 1 67i 0
2
0 1
151i 2 2142 1 334i 2 455i 0 0 1 82i
E B1 A E B1 A E
B:! E
A
MP2
+ ZPC
21.37 0.58 0.00 26.18 26.02 13.02 0.24 0.00 27.37 29.25 3.73 1.70 7.66 0.00 -7.10 0.42
MP2 21.46 -0.66
MP4 (SDTQ) -1.13 -222.52 725
-222.47381
15.58 14.53
QCISD(T, 26.32 -0.37
CCSD(T1
CCSD(T)+ ZPC
-0.54
-0.51
-222.48480
-222.4871 4
0.00
24.37 23.97
24.74 24.00
-0.26
24.93 24.93 13.21 -0.20
-0.29
41.01 35.75 12.93 -0.27
-1 72.31039
-1 72.33549
-1 72.33484
-1 72.33435
24.94 25.29 4.01 -0.19 8.63
24.04 23.49
24.12 23.02 4.38 -0.06 8.30
24.13 23.02
-0.22 0.00 26.13 25.48
-0.06
0.00
- 161.92499 -11.92 0.94
-1 61.94630
- 161.94584
-12.48 0.75
-11.73 0.71
0.06
-161.94510 -11.48 0.69
0.00
-11.68 0.92
ZPC was calculated at the MP2II level and scaled by 0.95. Total energies (italic, in hartree) are given only for reference structures, and relative energies (in kcallmol) for other structures are relative to the reference structures. The first entry is the number of imaginary frequencies, the second their values, and the third their symmetry.
frequency, is the transition state between one (mt) and another (mt). The (mt) structure will be able to scramble its hydrides relatively easily through the (b) transition state. The (m) structure at 3.7 kcaYmo1 above (mt), as well as (t)structure at 7.7 kcaYmo1 above (mt), has a doubly degenerate imaginary frequency and connects (mt) and (b) structures. The most remarkable feature of the PES for Au is that the global minimum is the hydrido(borane)gold, HAuBH3, structure (h), which is more stable than the (mt! AuBH4 structure by 7.1 kcaY mol at this level of theory. The transition state (ts) separating the AuBH4 (mt)structure from the HAuBH3 (h) structure is only 0.42 kcaYmol above the (mt)
structure; the (mt) structure is not likely to exist at this level of theory. The (ts) can structurally easily reach the reactant (mt) by moving Ht' onto the B atom. Pseudo-IRC following from the transition state (ts) to the product (h) indicates that the H atom to be transferred (Ht') has to move all the way from above the Au-B axis in (ts)in Figure 1to below the axis in (h)by making a 162" rotation around the Au atom, while the BH3 fragment makes a 52" conrotatory motion. Energetics at Higher Levels of Theory. Here we would like to discuss the energetics of critical structures of the present molecules calculated at higher levels of theory, MP4(SDTQ), QCISD(T1, and CCSD(T), in con-
3332 Organometallics, Vol. 14, No. 7, 1995
Musaev and Morokuma
Table 2. Calculated M-H, M-BH3, and M--B€&- Bond Dissociation Energies (in kcaumol) for MH, MBH3, and HMBHs Species with Various Basis Sets and Methods" molecule
basis set I MP2
MP2
basis set I1 QCISD(T) CCSD(T)
MP4 ( SDTQ)
CCSD(T) f ZPCb
exptlC 66.4 51.4 2 69.8 & 2
De(M-H) CuH AgH AuH HCuBH3 HAuBH~
49.0 36.7 57.0
54.2 44.2 67.0
62.1 48.0 69.6
46.4 48.5 69.6
57.8 48.4 69.5
60.1 50.5 72.4
52.8 40.6 66.6
57.3 45.7 82.0
68.3 48.7 84.3
207.1 48.5 82.8
59.0 48.5 87.8
62.9 51.9 87.6
7.6 1.5 4.1
12.4 6.9 10.1
18.2 7.6 10.4
-163.0 7.6 10.1
13.6 7.5 10.5
14.2 7.9 10.2
11.4 5.5 13.7
15.5 8.4 25.2
24.4 8.3 25.1
6.8 7.6 23.3
14.9 7.5 25.7
16.6 9.2 25.4
171.3 150.4 188.2
178.0 150.2 186.9
167.4 149.9 186.3
170.1 149.8 186.1
171.4 150.7 185.8
DdM-BH3 CuBH3 AgBH3 AuBH~ HCUBH~ HAuBH3
*
)
De(M'-BHdCUBH~ (b) AgBH4 (b) AuBH~(b)
At the MP2/I optimized geometries of the molecules (shown in Figure 1)and fragments (in Figure 2). ZPC at the scaled MP2II level. Experimental data were taken from ref 47.
Table 3. Transition Metal Valence s, p, and d Orbital Mulliken Gross Populations (MI,Mulliken Overlap Populations (Q), Atomic Charges (Z*),and Dipole Moments @) of MH, MBHs, HMCH3, and MB& (M = Cu, Ag, Au) Calculated at the HFiII Level molecule CuH AgH AuH CuBH3 AgBH3 AuBH~ HCuBH3 HAgBH3 HAuBH3 CuBH4(b) AgBH4(b) AuBH~(b) AuBH4(mt) a
S
P
d
0.75 0.72 1.08
0.15 0.11 0.06 0.12 0.01 0.04 0.34 0.07 0.26 0.27 0.15 0.25 0.15
9.95 9.94 9.77 9.97 9.99 9.94 9.91 9.95 9.56 9.96 9.99 9.89 9.81
0.68 0.83 0.88 0.78 0.78 1.01 0.32 0.27 0.56 0.91
Z;$
Q
M M-B
M-H'
M-Hb(M-Hb)
0.75 0.69 0.75 0.40 0.25 0.29 0.13 0.00 0.48 0.25 0.16 0.30 0.12
-.0.01 0.77 0.69 0.73
0.05 (0.17) 0.01 0.10 0.28 0.19 0.29 0.70
M f0.15 f0.23 +0.09 +0.23 +0.17 +0.14 fO.O1 +0.20 f0.17 f0.45 f0.59 +0.30 +0.13
B
Ht'
Hb(Hb')
-0.11 -0.06
-0.03 -0.04
-0.05 -0.04
+0.19 f0.13 -0.14 -0.16 -0.18 -0.32 -0.02
-0.20 -0.30 -0.11
0.0 (+0.02) -0.04 +0.07 -0.09 -0.14 +0.03 (0.0)(-0.04)
-0.04"
u (D)
4.08 4.34 2.60 2.95 1.94 1.73 5.45 5.41 1.92 6.25 7.65 5.51 4.26
For the Ht atom.
junction with the larger basis set I1 with ZPC calculated from the scaled MP2A frequencies. In these calculations we used the geometries optimized at the MPBA level. As seen in Table 1, at the highest level of theory, CCSD(T)/II + ZPC, for CuBH4 and AgBH4 the (b) and (h) structures are stabilized a few kcallmol more than the (t)structure than at the MPBA ZPC level. As a result, the (b) structure becomes 0.51 and 0.22 kcallmol, respectively, more stable than the (t)structure. The (ts) structure is stabilized a few kcal/mol more than the (h) structure; the HMBH3 structure (h) lies 0.74 and 0.65 kcallmol, respectively, higher than the (ts) separating of the (h) structure from (t)or (b) for CuBH4 and A g B a . However, the main conclusion made above at the MP2 level does not change; CuBH4 and AgBH4 molecules exist only as tetrahydroborate species, MBH4, with nearly degenerated (b) and (t) structures. They are nonrigid and at ambient temperature should exchange bridge and terminal hydrogen atoms very easily by the pathway (b) * (t)t (b') t (t') t .... AgBH4 is still a little more fluxial than CuBH4. The hydrido(borane1metal HMBH3 (h) structure is thermodynamically unfavorable by 25 and 26 kcallmol for CuBH4 and AgBH4,
+
respectively, and kinetically unstable or only marginally stable; it is not likely t o exist. For the interest of theoreticians, here we would like t o discuss briefly the total failure of the QCISD(T) method for copper complexes. As seen in Table 1, the calculated energy difference between (t)and (h) structures of CuBH4 jumps to 41 kcallmol at the QCISD(T) level from 15-20 kcallmol at the other levels. As shown in Table 2, the HCu-BH3 and Cu-BH3 bonding energies calculated at the QCISD(T1level are 150-200 kcall mol different from those obtained at the other levels. Note this erratic behavior of QCISD(T)arising from the (T) correction term has been recognized recently elsewhere,46and the user of the method should be warned of this problem. For AuBH4 at the highest CCSD(T) ZPC level of theory the (b) and (mt)AuBH4 structures are degenerate, and it is difficult to conclude which of them is energetically more stable. They are 11.7 kcallmol higher than the hydrido(borane)gold complex HAuBH3 (h),and they rearrange into (h) with a small barrier of 0.92 kcaymol, a t the best level. The barrier height is
+
1461 Bohme. M.; Frenking, G. Chem. Phys. Lett. 1994,224. 195.
CuBH4, AgBH4, and AuBH4
'""j
Organometallics, Vol. 14, No. 7, 1995 3333
r
AEA ( kcal/mol)
H + AgBHj
90.0
the trend predicted earlier.48q49As also seen in Table 2, the dissociation energy of the tetrahydroborate MBfi (b) structure to M+ BH4- increases in the same order as the M-H bond energy in the MH molecules, i.e. AgBH4 (151 kcdmol) < C u B h (171 kcdmol) < A u B h (186 kcallmol). According to the energy diagram given in Figure 3, the thermodynamically favorable channel of MBH4 decomposition is MH BH3 for all metals considered. However, the mechanism of this process is different for M = Cu and Ag and for M = Au. Since CuBH4 and AgBH4 exist only as tetrahydroborate species, the reaction MBH4 MH BH3 should takes place in two steps; at fist, they need 24.7 and 26.1 kcdmol of energy for M = Cu and Ag, respectively, t o rearrange into (metastable) the hydrido borane structure, HCuBH3 and HAgBH3, from where the dissociation into MH and BH3 fragments requires an additional 16.6 and 9.2 kcal/mol. The overall energy needed for decomposition of CuBH4 and AgBH4 into the MH and BH3 fragments is large, 41.3 and 35.3 kcdmol for Cu and Ag, respectively. On the other hand, the AuBH4 complex exists only as a hydrido borane species, HAuBH3, and the decomposition reaction takes place by one step and leads to AuH and BH3 products with an endothermicity of 25.4 kcaymol. Thus, our calculations show that the thermal stability of the CuBH4, AgBH4, and AuBH4 species increases in the order AuBH4 (25.4 kcal/mol) < AgBH4 (35.3 kcal/ mol) < CUB& (41.3 kcal/mol), which is consistent with available experiment^.^^ The difference in fundamental features of the potential energy surface, i.e. the difference in reactivity of Cu+ and Ag+, on one side, and Au+, on the other side, with BH4- is directly related t o the differences among first-, second-, and third-row transition metals in binding energies DJM-H), De(M-BH3), De(HM-BH3), and De(H-MBH3) discussed above. The metal-ligand bonding energy in general changes in the following order: first row > second row < third The fact that the metal-ligand bond of the first-row transition metal is stronger than that of the second row is often explained in terms of larger overlap of the first-row metal 4s orbital with the H 1s or ligand sp hybrid orbital than the second-row 5s orbital. The expectation value of radius (r)of the Cu 4s orbital is 1.92 A, compared with 1.64 A of Ag 5s and 1.56 A of Au 6s, and the larger size On of Cu 4s provides a better overlap with the other hand, the fact that the metal-ligand bond of the third-row transition metal is stronger than that of the second row is explained in terms of the magnitude of d orbital contribution in bonding. At first, for heavier atoms sldn+land the sodnf1states are energetically not very much higher, and sometimes even lower, than the s2dnstates. In addition, the size of the d orbital, relative to the s orbital, increases as the metal becomes heavier. These factors result in better s-d hybridization and larger d involvement in the bonding of heavier transi-
+
CuBH3
70.0 *O*,j
H + AuBH3
+
BH3 + CUH 20.0
-
0.0-
BH3 + AuH -30.0
HAuBH~
Figure 3. Relative energies hErelof the most stable MBH, structure, the transition state (ts), HMBH3, H + MBH3, and MH + BH3 (where M = Cu, Ag, and Au) calculated at the CCSD(T)/II + ZPC (scaled M P m ) level at the M P m optimized geometries. The energy origin is arbitrarily shified for each M. quite small, and it is unlikely that any AuBH4 structure, (mt) or (b), exists. Thus, one may conclude that the reactivity of Cu+, Ag+, and Au+ cations with the BH4anion would be very different; the reaction of Cu+ and Ag+ with BH4- leads to the tetrahydroborate complexes CuBH4 and AgBH4, respectively, while the same reaction of Auf yields hydrido borane complex HAuBH3. Our conclusion is in good agreement with available experiment~.~~-~~ As seen in Table 2, the calculated M-H bonding energies at our best CCSD(T) ZPC level are 60.1,50.5, and 72.4 kcaumol for the bare metal hydrides CuH, AgH, and AuH, respectively. The calculated numbers for M = Ag and Au are in reasonable agreement with experimental values4' of 51.4 f 2 and 69.8 f 2 kcal/ mol, respectively. The value for M = Cu is about 6 kcal/ 66.4 kcal/ mol smaller than the experimental mol, which may be a result of the absence of the relativistic effects in our calculation for M = Cu. Addition of the BH3 ligand to the metal atom M strengthens the H-M bond by 2.8, 1.4, and 15.2 kcal/ mol for M = Cu, Ag, and Au, respectively, and as a result, the M-H bond in HMBH3 structure becomes 24.7 and 35.9 kcaumol stronger for Au than for Cu and Ag, respectively. While the calculated strengths of the M-BH3 (14.2, 7.9, and 10.2 kcal/mol for Cu, Ag, and Au, respectively) and HM-BH3 (16.6,9.2,and 25.4 k c d mol for Cu, Ag, and Au, respectively) bonds in MBH3 and HMBH3, respectively, are significantly less than the metal-hydride bond, the HM-BH3 bond for Au is about 8.8 and 16.2 kcaYmol stronger than for Cu and Ag, respectively. Thus, these results show that the M-X bonds (especially the M-H bond) are stronger for the first-row transition metal Cu than for the second-row Ag and that those for the third-row transition metal Au are the strongest of all, which is in good agreement with
+
(47) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure. N . Constants of diatomic molecules; Van Nostrand Reinhold Co.: New York, 1979.
+
(48)Walch S. P.; Bauschlicher, C. W., Jr. In Comparison ofAb Initio Quantum Chemistry with Experiment; Bartlett, R., Ed.; Reidel: Boston, MA, 1985; p 17. (49)Langhoff, S. R.; Pettersson, L. G. M.; Bauschlicher, C. W., Jr.; Partridge, H. J . Chem. Phys. 1987,86, 268. (50) Schilling, J. B.; Goddard, W. A.. 111; Beauchamp, J. L. J . Am. Chem. Soc. 1986,108, 582. (51)Schilling, J. B.; Goddard, W. A,, 111; Beauchamp, J. L. J . Am. Chem. SOC.1987, 109, 5565. (52)Ohanessian, G.; Brusich, M. J.; Goddard, W. A., 111. J . Am. Chem. SOC.1990, 112, 7179.
3334 Organometallics, VoE. 14, No. 7, 1995
tion metals. The relativistic effects, which cause the above differences and increases for heavier metals, have been explicitly evaluated in theoretical studies. For example, Schwerdtfeger et al.53 have shown that the inclusion of relativistic effects decreases the Au-H bond length by 0.25A and, consequently, increases the Au-H binding energy as much as 15 kcallmol. Mulliken Population Analysis. The above discussed trends of bond energies can be related t o the results of the Mulliken population analysis. As shown in Table 3, the metal atom in CuH, CuBH3, AgH, and AgBH3 molecules has a Mulliken population of d9.95s0.75p0.15,d9.97s0.68p0.12,d9.94s0.72p0.11,and d9.99s0.83p0.01, respectively. The d electrons are not significantly involved in the bonding. Thus R(M-X) and De in these molecules are mainly determined by the overlap of the valence s orbital with the ligand. The radial overlap of the Cu 4s orbital with the H 1s orbital or B sp hybrid orbital is slightly better than the overlap of the Ag 5s orbital, as reflected in the difference in overlap population Q in Table 3,and consequently De is slightly larger in CuH and CuBH3 than in AgH and AgBH3, respectively. In contrast, the Mulliken population of Au in AuH and AuBH3 is d9.77s1.08p0.06 and d9.9490.88P0'0 4 , respectively, and the d-electrons of Au are more strongly involved in bonding than those of Cu and Ag, which makes the Au-H and Au-BH3 bonds Merent in nature and stronger compared with their Cu and Ag analogs. As was discussed above, extra H and BH3 ligands on the transition metal atom strengthens the metal-BH3 and metal-H bonds, respectively. Though these changes are small, 2-3 kcallmol, for Cu and Ag, they are quite large, about 15 kcdmol, for Au. Similar results have been found for MCH3+ and M(CH3)2+ of first- and second-row transition metal atoms by Rosi et al.54and for MH+, MCH3+, and HMCH3+ of Fe, Co, Rh, and Ir by The origin of this effect is that the first ligand facilitates the involvement of low-lying dn+Wand dnslpl electronic configurations of the metal atom into the M-X bond, thus increasing the p and d character of the bond, and consequently makes the second bond stronger than the first. As seen in Table 3, the Mulliken population of metal atoms in HMBH3 complexes of d9.91s0.78p0.34,d9.95s0.78P0.07,and d9.56S1.01p0.26,respectively, (53)Schwerdtfeger, P.; Dolg, M.; Schwarz, W. H. E.; Bowmaker, G . A,; Boyd, P. D. W. J. Chem. Phys. 1989,91, 1762. (54) Rosi, M.; Bauschlicher, C. W., Jr.; Langhoff, S. R.; Partridge, H.J. Phys. Chem. 1990,94, 8656. ( 5 5 )Musaev, D.G . ;Koga, N.; Morokuma, K. J . Phys. Chem. 1993, 97, 4064. (56)Musaev, D.G.; Morokuma, IC;Koga, N.; Nguyen, K. A.; Gordon, M. S.; Cundari, T. R. J. Phys. Chem. 1993, 97, 11435. (57) Musaev, D. G.; Morokuma, K. Isr. J . Chem. 1993,33, 307.
Musaev and Morokuma for M = Cu, Ag, and Au substantiates the above arguments.
Conclusions The following key points may be drawn from the calculations presented here. 1. The CuBH4 and AgBH4 molecules exist only as tetrahydroborate species, MBH4. The lowest geometrical structures are (b) and (t) structures, of which the (b) structure is at the best level (CCSD(T) ZPC) of theory, slightly (0.5and 0.2 kcallmol for CuBH4 and AgBH4, respectively) more stable than the (t)structure. The monodentate structure (m) lies about 20-25 and 13 kcallmol higher than (b) or (t)structures >forCuBH4 and AgBH4, respectively. For these molecules the HMBH3 structure lies about 25-26 kcallmol higher than the most stable MBH4 structure and does not exist as an intermediate but rearranges into the tetrahydroborate structure, MBH4, with a very small (or no) barrier. 2. The corresponding complex of Au, AuBH4, exists only as hydrido(borane)gold, HAuBH3. Its tetrahydroborate structure, about 12 kcallmol higher, does not exist and rearranges to the hydrido(borane)gold, HAuBH3, structure with a small barrier. 3. At the best level (CCSD(T)+ ZPC) of theory the bond energies are calculated to be 60.1,50.5,and 72.4 kcallmol for the M-H bond, 14.2, 7.9,and 10.2 kcall mol for the M-BH3 bond, 62.9,51.9,and 87.6 kcdmol for the H-MBH3 bond, 16.6,9.2,and 25.4 kcallmol for the HM-BH3 bond, and 171.4,150.7,and 185.8 kcall mol for the M+-BH4- bond, respectively, for M = Cu, Ag, and Au. 4. The thermal stabilities of CuBH4, AgBH4, and AuBH4 complexes are calculated to be 41.3,35.3,and 25.4 kcdmol, i.e., decreases in the order CuBH4 > AgBH4 > AuBH4, which is in good agreement with available e ~ p e r i m e n t . ~The ~ - thermodynamically ~~ most favorable decomposition channel of the MBH4 molecule is MH BH3 for all metals considered. It takes place by a two-step mechanism for the CuBH4 and AgBH4 species; at first the stable tetrahydroborate structure, MBH4, rearranges into hydrido borane, HMBH3, which dissociates into HM and BH3 fragments. However, for AuBH4 species the decomposition process takes place by a one-step mechanism, simple dissociation of the stable structure HAuBH3 t o AuH and BH3 fragments.
+
+
Acknowledgment. The present research is in part supported by the Grant CHE-9409020from the National Science Foundation. OM9501539
