The Road to 13â15 Nano Structures: Structures ... - ACS Publications

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J. Phys. Chem. A 2010, 114, 516–525

The Road to 13-15 Nano Structures: Structures and Energetics of (MYH2)4 Tetramers (M ) B, Al, Ga; Y ) N, P, As) Alexey Y. Timoshkin* Inorganic Chemistry Group, Department of Chemistry, St. Petersburg State UniVersity, UniVersity Pr. 26, Old Peterhof, 198504 Russia

Henry F. Schaefer III Center for Computational Quantum Chemistry, UniVersity of Georgia, Athens, Georgia 30602 ReceiVed: August 1, 2009; ReVised Manuscript ReceiVed: September 17, 2009

AIIIBV compounds are important for the development of advanced ceramic semiconductor and nanoelectronic materials, solar cell elements, and light-emitting diodes. A series of these group 13-15 compounds of the general formula M4Y4H8 (M ) B, Al, Ga; Y ) N, P, As) has been theoretically studied at the B3LYP/TZVP level of theory. The stability of different isomer structures is discussed to reveal the competitiveness of group 13-13, group 13-15, and group 15-15 bonding. Preferential bonding patterns are established, and trends in the stability with respect to M and Y are also discussed. New structural types, featuring perfectly planar M4 and Y4 rings, have been established. 1. Introduction Oligomeric group 13-15 imido compounds [RMNR′]n and their phosphorus and arsenic analogs are well-known laboratory species.1-4 In the past decade, they have attracted attention as precursors to 13-15 binary and composite materials5-9 as well as important intermediates in hydrogen storage technology.10-13 Poly(iminoboranes) have been identified among the products in dehydrogenation reactions in the ammonia-borane system.14-18 Group 13 metal (Al, Ga) imido compounds form cage structures, whereas boron compounds adopt ring structures, with the oligomerization degree in both cases strongly dependent on the size of the substituents.4 In recent studies, it has been shown that low valent group 13 compounds (like gallium(I) amide, GaNH2) may play a significant role in 13-15 CVD.19-21 Recent developments in low valent group 13 chemistry have resulted in isolation and structural characterization of 13-13 donor-acceptor complexes22-29 and 13-13 bonded metalloid clusters.30-37 Theoretical studies of 13-13 donor-acceptor complexes38 show the exceptional stability of boron-boron bonded compounds. The smallest metalloid cluster Al4(N(SiMe3)2,6-iPr2C6H3)4, structurally characterized by Roesky in 2003,37 features an Al4 core bearing amido NRR′ ligands. As pointed out by one of the reviewers, the strong AlN bond in [AlNR2]4 species is especially evident in the partially substituted compound Al4Cp3N(SiMe3)2.31 In 2007, two similar gallium [GaNRR′]4 compounds were reported by Linti.35 Formally, these compounds are structural isomers of the wellknown tetrameric [RMNR′]4 imido compounds.4 Very recently, the tetrameric metalloid cluster [Al4(PtBu2)6] featuring two additional PtBu2 bridging groups has been reported by Schno¨ckel et al.32e Therefore, there are experimental examples of the realization of tetramer compounds featuring both 13-15 and 13-13 cluster cores. Knowledge of the preferential bonding patterns is of fundamental importance for the construction of precursors for 13-15 CVD and for designing new materials for hydrogen storage. * Corresponding author. E-mail: [email protected].

To address this problem in a systematic way, a theoretical study of the series of compounds with formal composition [MYH2]n (M ) B, Al, Ga; Y ) N, P, As), which may be considered to be built up from n MYH2 monomer units, has been undertaken. Results for monomeric and dimeric compounds have previously appeared.39 In the present report, structures and relative stabilities of tetrameric compounds are discussed. In line with our previous report,39 structures of tetramers are constructed on the basis of MYH2, HMYH, and H2MY bonding patterns. Cage and ring isomers for MYH2 and H2MY bonding patterns and cubanetype, ladder, and ring isomers for the HMYH bonding pattern have been considered. A qualitative representation of the structures examined is given in Scheme 1. 2. Computational Details All structures were fully optimized and verified with subsequent vibrational analyses to be minima on their respective potential energy surfaces (PESs). Density functional theory in the form of the hybrid B3LYP functional40,41 was used together with the all-electron triple-ζ quality TZV basis set of Ahlrichs, augmented by d-type polarization functions (TZVP).42 For H, the standard 6-311G** basis set was employed. The Gaussian 94 suite of programs43 was used throughout. Previously, this level of theory was successfully used to study six-member inorganic heterocycles [BAlGaNPAs]H6 and their dimers.44 As shown for [HAlYH]n oligomers (Y ) N, P; n ) 1-4),45 results from the B3LYP level of theory give quite satisfactory agreement with those obtained at the CCSD(T)/cc-pVTZ level. 3. Results and Discussion 3.1. [MYH2]4 Compounds. Solid-state structures of the tetrahedral M4L4 clusters of group 13 metals are well-established experimentally.32-37 However, only three structurally characterized compounds with an M4 core featuring amido ligands NRR′ are known at the present time. Previous theoretical studies of group 13 metal clusters with group 15 element substituents are also scarce. Structural data for S4 symmetric Al4(NH2)4 were

10.1021/jp907410h  2010 American Chemical Society Published on Web 10/20/2009

The Road to 13-15 Nano Structures

J. Phys. Chem. A, Vol. 114, No. 1, 2010 517

SCHEME 1: Qualitative Structures of the Tetramer M4Y4H8 Compounds Studied in the Present Work

earlier obtained by Roesky and coworkers at the SCF/SV(P) level of theory.37 For the series of Ga4(NR2)4 compounds, substitution effects have been discussed in a recent joint experimental-theoretical study by Linti.35 However, in both cases, no energetic comparisons with the respective cubanetype or other possible isomers were made. Our optimized tetrahedral structures of [MYH2]4 clusters (S4 point group) are true minima on PES for Y ) P, As (Figure 1d-i). For the tetrahedral clusters, bearing NH2 substituents, structures of S4 symmetry are predicted to be transition states (TS), with one soft imaginary mode. To check if this result is an artifact of the used computational approach, computations using a finer integration grid (99 radial shells, 590 angular points per shell) were performed and yielded identical results. Optimization without symmetry constraints (in the framework of C1 point group) resulted in the structures shown in Figure 1a-c, which are respectively only 0.1, 1.5, and 1.4 kJ mol-1 lower in energy than the S4 symmetric TS for M ) B, Al, and Ga. Optimized structures of S4 symmetric M4(YH2)4 for Y ) P, As feature nearly perfect tetrahedral M4 cores for all group 13 elements. (The maximum difference in M-M bond lengths is less than 0.012 Å.) On the contrary, for the Y ) N asymmetric minima exhibit distinctly inequivalent M-M distances. The largest difference between M-M distances increases in the order B (0.043 Å) < Al (0.108 Å) < Ga (0.159 Å), suggesting that the Ga4 core is the most distorted, which is in agreement with the previous theoretical findings by Linti.35 Note that the experimentally known Ga(I) amido compounds [Ga(tmp)]4 and [Ga{N(SiMe3)(dipp)}]4 (tmp: 2,2,6,6-tetramethylpiperidino; dipp: 2,6-iPr2C6H3) also exhibit a considerably distorted tetrahedron core (with largest Ga-Ga bond distance difference of 0.1 Å), in contrast with nearly perfect silyl- and organyl-substituted gallatetrahedranes.35 Note that neither bulky organic and siliconbased substituents nor PH2 and AsH2 groups influence significant core distortion. Therefore, it appears that it is the amido NH2 group that induces the strongest distortion of the M4 core in

tetrahedranes. This may be a result of a preference for stronger π-bonding in the monomeric GaNH2 molecule. We must note that optimization attempts for the unbridged [MYH2]4 ring isomers (Scheme 1b) always resulted in cage compounds featuring tetrahedral M4 units (Scheme 1a). However, for the YH2 bridged [MYH2]4 ring isomers (Scheme 1c), the structures obtained are true minima on PES for all 13-15 pairs but BAs. These C2h symmetric molecules feature a perfectly planar rhombus of group 13 elements (Figure 2). There are two examples of donor-stabilized aluminum monohalides adopting the planar Al4 core: [AlBr · NMe3]432f and [AlI · NMe3]4.32g For BAs, the C2h symmetric structure is predicted to be a TS with an in-plane imaginary mode. Optimization without symmetry constraints resulted in a rather asymmetric structure, in which one of the bridging AsH2 groups shifts toward the boron atom. Note that all four boron atoms in the final asymmetric structure of [BAsH2]4 (Figure 2c) are nearly coplanar (dihedral BBBB angle is only 1.3°). The comparison of the relative energies between structures with tetrahedral and planar M4 cores (Table 1) reveals that for M ) B, the tetrahedral structures are by about 230-390 kJ mol-1 lower in energy compared with C2h symmetric geometries with the planar M4 core. In contrast, for the Al and Ga compounds, these isomers lie much closer in energy. The favorability of the planar structures increases in the order As < P < N; Al < Ga. As a result, for GaN, AlN, and GaP systems, the planar M4 rhombic structures are more favorable (by 121, 21, and 17 kJ mol-1, respectively). Nevertheless, tetramers with the [MYH2]4 bonding pattern are always higher in energy compared with cubane-type structures with the [HMYH]4 bonding pattern (Table 1). The dissociation energies of tetrahedral [MYH2]4 structures into monomers (Table 2) decrease in the order

BAs g BP > BN . AlAs g AlP > AlN g GaAs g GaP > GaN

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Figure 1. Optimized structures for tetrahedral [MYH2]4 clusters: (a) [BNH2]4, (b) [BPH2]4, (c) [BAsH2]4, (d) [AlNH2]4, (e) [AlPH2]4, (f) [AlAsH2]4, (g) [GaNH2]4, (h) [GaPH2]4, (i) [GaAsH2]4. All distances in angstroms, angles in degrees. B3LYP/TZVP level of theory.

Boron compounds have the largest dissociation energies, whereas the tetrameric gallium amide is the weakest bound. Our predicted dissociation energy of 125 kJ mol-1 for [GaNH2]4 is higher than the 56 kJ mol-1 obtained by Linti35 with the RIDFT BP86/def-SVP method. Note that our computational results predict that [AlNH2]4 has a dissociation energy of 279 kJ mol-1 and is significantly strongly bound compared with [GaNH2]4. This is in good agreement with qualitative experimental observations. In the electron ionization mass spectra, no molecular peaks were detected for Ga derivatives,35 whereas the Al compound showed a molecular ion peak with relative intensity of 16%.37 In addition, the dissociation of Ga compounds in deuterobenzene solution at 60 °C was clearly evident.35 In contrast, no clear evidence of the dissociation of Al compound in deuterobenzene solution at 70 °C was given, although it was mentioned that dissociation can not be ruled out.37 3.2. [HMYH]4 Compounds. 3.2.1. Cubane Compounds. For the RMYR bonding pattern, cubane species are the most abundant among the structurally characterized [RMYR]n compounds.4 They have also been extensively studied theoretically. Recent examples include case studies of B-N,44,46 Al-N,47-49 Ga-N,50-52 and Al-P53 systems as well as comparative studies of halide-substituted45 group 13 metal derivatives [XMYH]4 (X

) F, Cl, Br, I) and mixed metal and mixed pnictogen cubanes of the types [AlxGayIn4-x-yY4H8] and [M4NxPyAs4-x-yH8].54 In agreement with previous theoretical findings,46-53,55-59 all [HMYH]4 cubanes adopt highly symmetric structures (Td point group, Figure 3). Dissociation of the cubanes into HMYH monomers (Table 2) requires significant energy. The order of dissociation energies is

AlN > GaN . AlP > AlAs > GaP > GaAs > BP > BN > BAs Among Al and Ga species, nitrogen-containing cubanes have exceptional stability toward dissociation into monomers. Boroncontaining cubanes have the lowest dissociation energies. Nevertheless, the absolute value for the dissociation energy of the weakest bound [HBAsH]4 cubane into monomers is 450 kJ mol-1, which is significantly larger than the weakest bound tetrahedrane [GaNH2]4. Therefore, cubane-type compounds should not dissociate into monomers. However, dissociation of boron-containing cubanes into the most stable [HBYH]2 dimers is endothermic by only 64, 125, and 35 kJ mol-1 for Y ) N, P, and As, respectively.



Figure 2. Optimized structures for ring [MYH2]4 clusters: (a) [BNH2]4, (b) [BPH2]4, (c) [BAsH2]4, (d) [AlNH2]4, (e) [AlPH2]4, (f) [AlAsH2]4, (g) [GaNH2]4, (h) [GaPH2]4, (i) [GaAsH2]4. All distances in angstroms, angles in degrees. B3LYP/TZVP level of theory.

TABLE 1: Relative Energies, E, in kilojoules per mole (0 K, without ZPE Corrections) of the Tetrameric Species with Respect to the Cubane [HMYH]4 Structure (B3LYP/TZVP Level of Theory)a [MYH2]4

[HMYH]4

[H2MY]4

M,Y

tetrahedral

ring

cube

ladder

ring

ring

point group

S4

C2h

Td

Ci

D4h, D2d

D4h, D2d

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

-40.9 -22.8 -55.2 196.8 94.7 62.0 163.3 59.6 29.9

-268.2 -102.7e -102.2e 234.9c 112.7e 67.1e 171.5c 30.3e -7.0e

c

BN BP BAs AlN AlP AlAs GaN GaP GaAs

b

311.8 116.2 60.8 568.7b 316.7 289.8 219.9b 115.3 107.7

703.1 515.7 291.6d 542.2 338.8 365.8 97.1 97.4 132.9

a Only energies for structures that are true minima on their PES are reported. point group. f D2 point group.

3.2.2. Ladder-Type Compounds. Ladder-type structures were proposed by Paetzold1 and Gilbert60 as intermediates for the dimerization of dimeric iminoboranes to yield eight-membered ring structures. Formally, breaking two of the 13-15 bonds in the cubane core and subsequent structural rearrangement (“opening up”) should result in ladder compounds. However, as was noted by Gilbert, such transformations would require the transannular M and Y centers to adopt nearly square-planar conformations, which are highly unfavorable energetically. It is probably the rigidity of the structure that allows the synthesis and isolation of ladder compounds for heavier group 13 elements.4,61-63 Because all ladder compounds feature two terminal pnictogen and two terminal group 13 atoms with low coordination number 3, such unsaturated terminal atoms are expected to be active toward donor and acceptor molecules. In fact, all structurally characterized 13-15 ladders are supported by two donor ligands.61-63 Unsupported ladders adopt cen-

b

c

bridged 1005.2d -99.3 -256.0 1336.3d 303.9 152.2 1111.7d 249.6 112.4

734.1 68.0e -88.2e 1363.2f 1056.8f

C1 point group. c D4h point group.

D2h

d

C2 point group. e D2d

TABLE 2: Dissociation Energies of the Tetrameric Structures into Monomers: ∆E° in kilojoules per mole (0 K, without ZPE Corrections) (B3LYP/TZVP Level of Theory)

M,Y

tetrahedral[MYH2]4 ) 4 MYH2

ring[MYH2]4 ) 4 MYH2

cube[HMYH]4 ) 4 HMYH


895.9 1326.1 1329.4 279.0 402.2 404.9 124.5 266.6 271.7

504.7 926.6 1098.6 307.1 380.2 328.9 248.7 284.5 246.5

513.6 565.2 452.6 1581.2 909.1 788.6 1190.6 677.9 589.7

trosymmetric structures (Ci point group). Optimized structures are given in Figure 4. It must be noted that a recent experimental

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Figure 3. Optimized structures for cubane [HMYH]4 compounds: (a) [HBNH]4, (b) [HBPH]4, (c) [HBAsH]4, (d) [HAlNH]4, (e) [HAlPH]4, (f) [HAlAsH]4, (g) [HGaNH]4, (h) [HGaPH]4, (i) [HGaAsH]4. All distances in angstroms, angles in degrees. B3LYP/TZVP level of theory.

study showed that gallium- and indium-based cubanes [RMPR′]4 are stable with respect to reaction with such Lewis bases as pyridine.64 Bonds between group 13-15 elements in ladders are very diverse. Note the very large difference in MY bond distances formed by terminal and core atoms. The longest bond in all ladder structures is formed by the core atoms, which are in the middle of the ladder. With respect to cubanes, in the central M2Y2 ring, one MY bond is elongated (between two “zigzag” MYMY chains) and the other (inside the MYMY chain) is shortened. In all ladders, terminal atoms of group 13 elements adopt virtually planar geometries (sum of angles around M is close to 360°). Note that for nitrogen-containing ladder compounds, the M and N atoms in all three four-member M2N2 rings are virtually coplanar (maximal dihedral MNMN angle is 7.1 for M ) B, and 6.0 for M ) Al, Ga). In contrast, for P, As ladders, the terminal rings are highly puckered, resembling the strong puckering for the individual [HMYH]2 dimers of C2V symmetry in the case of the P, As compounds.39 It should be noted that terminal P and As atoms adopt highly distorted (pseudotetrahedral) structures. This possibility for major structural relaxation leads to the fact that despite breaking two 13-15 bonds, relative energies of (P, As)-containing ladders with respect to cubane isomers are not very high (30-95 kJ mol-1). Because of the large difference in covalent radii of the B and As atoms, the [HBAsH]4 ladder structure (Figure 4c) is highly distorted. The boron atoms lie in close proximity to each other: the optimized B-B distance is 1.893 Å, as compared with 2.091 and 1.979 Å for ladder-[HBNH]4 and ladder[HBPH]4, respectively. These B · · · B distances can be compared

with those for the C2V puckered dimers (1.902, 2.110, and 2.159 Å for [HBNH]2, [HBPH]2, and [HBAsH]2, respectively).39 However, those distances are still significantly longer than those in tetrahedral [BYH2]4 (1.679 to 1.688 Å) and in B-B bonded 13-15 donor-acceptor complexes (1.684 to 1.689 Å).38 For all boron-containing compounds, ladder structures are lower in energy compared with the cubane isomers. 3.2.3. Ring Isomers. Breaking the two longest 13-15 bonds in the ladder results in an opening up of the structure and formation of ring isomers. Optimized structures of the most stable ring isomers are given in Figure 5. Nitrogen-containing [HMNH]4 rings (Figure 5 a,d,g) adopt perfectly planar highly symmetric structures (D4h point group). According to Gilbert,60 [HBNH]4 does not obey Hu¨ckel’s rule and has a delocalized π-system similar to that of borazine. In contrast, phosphorus- and arsenic-containing rings are nonplanar. Optimization attempts of (P, As)-containing rings in the framework of structures with the main C4 axis either did not yield a minimum on the PES or lead to high-energy structures. Lowering the symmetry allowed us to obtain D2d symmetric ring structures (Figure 5), which are true minima on PES. These compounds feature a main S4 axis and a highly distorted M4Y4 ring. Group 13 elements adopt trigonal planar environments, whereas the P and As atoms are pyramidal. Alternative ring structures of C4V and C2 symmetry (Figure 1S, Supporting Information) were found to lie 14-28 kJ mol-1 higher in energy compared with the analogous D2d symmetric rings (Table 3S, Supporting Information). Despite the fact that the cubane structure [HBNH]4 is a minimum on the PES and its formation either from HBNH monomers44,46 or from [HBNH]2 dimers is exothermic, the



Figure 4. Optimized structures for ladder [HMYH]4 isomers: (a) ladder-[HBNH]4, (b) ladder-[HBPH]4, (c) ladder-[HBAsH]4, (d) ladder-[HAlNH]4, (e) ladder-[HAlPH]4, (f) ladder-[HAlAsH]4, (g) ladder-[HGaNH]4, (h) ladder-[HGaPH]4, (i) ladder-[HGaAsH]4. All distances in angstroms, angles in degrees. B3LYP/TZVP level of theory.

formation of ladder-type and especially the D4h symmetric ring structure is even more energetically favorable. In contrast, for the Al and Ga analogs, the situation is reversed. Although cyclic D4h symmetric [HMNH]4 structures (M ) Al, Ga; Figure 5d,i) are also true minima on PES, they lie much higher in energy compared with the cubane type isomers.50,57 These results are TABLE 3: Dissociation Energies of the Tetramers into Dimers: ∆E° in kilojoules per mole (0 K, without ZPE Corrections) (B3LYP/TZVP Level of Theory) M,Y

ring-[MYH2]4 ) 2 ring-[MYH2]2

bridging ring [H2MY]4 ) 2 rhomb-[H2MY]2


642.0a 314.8 691.3 147.1 218.2 199.0 91.1 131.1 119.3

-18.5 324.9 314.7 -191.2 153.3 168.8 -168.7 190.8 205.4

a

Result for the rhombic [MYH2]2 dimer.

in qualitative agreement with the experimental observations that boron-containing tetramers exist as cyclic ring species with coordination number 3 on boron and nitrogen centers,1 whereas group 13 metal derivatives adopt cubane type structures.4 It is interesting to note that whereas B-containing ring compounds are always lower in energy compared with both cubane-type and ladder structures, Al-containing rings are higher in energy than both cubane-type and ladder structures. The situation for gallium is unique because for the GaP and GaAs derivatives, ring structures are predicted to be lower in energy compared with ladder structures. Moreover, the transformation of [HGaAsH]4 cubane into a D2d symmetric ring is predicted to be exothermic by 7 kJ mol-1. This results from the apparently lower stability of Ga-As bonds in cubane44 and high favorability of the pyramidalized arsenic center.65 Therefore, despite breaking four Ga-As bonds (which are the weakest among 13-15 pairs), favorable structural relaxation (pyramidalization of two As atoms) fully compensates these expenses and even slightly favors the D2d symmetric ring structure for [HGaAsH]4. Although the D2d symmetric rings theoretically predicted in the present research are yet unknown experimentally, their isolation may become feasible by additional donor-acceptor

522



Figure 5. Optimized structures for ring [HMYH]4 isomers: (a) ring-[HBNH]4, (b) ring-[HBPH]4, (c) ring-[HBAsH]4, (d) ring-[HAlNH]4, (e) ring[HAlPH]4, (f) ring-[HAlAsH]4, (g) ring-[HGaNH]4, (h) ring-[HGaPH]4, (i) ring-[HGaAsH]4. All distances in angstroms, angles in degrees. B3LYP/ TZVP level of theory.

stabilization, for example, interaction with Lewis bases, similar to those experimentally realized for ladder structures by von Ha¨nisch.61-63 3.3. [H2MY]4 Compounds. Optimization of the unbridged ring isomers resulted in the structures presented in Figure 6. The perfectly planar structure of D4h symmetry is predicted to be a true minimum only for [H2BN]4. For other 13-15

pairs, D2d or D2 symmetric structures are minima on their PES. The N4 ring in [H2AlN]4 and [H2GaN]4 structures is puckered. Compounds featuring N4 rings are predicted to be highly energetic (>1000 kJ mol-1 above the most stable isomers). We also considered MH2 bridged structures (Figure 7). These are formed by joining two [H2MY]2 rhombic dimers. (See

TABLE 4: Preferential Bonding Patterns (in Bold) for the Monomeric, Dimeric, and Tetrameric Compounds of [MYH2]n Compositiona n)1 M,Y BN BP BAs AlN AlP AlAs GaN GaP GaAs a

n)2

n)4

lowest isomer

second lowest

E

lowest isomer

second lowest

E

lowest isomer

second lowest

E

HMYH (C∞V) HMYH (Cs) HMYH (Cs) MYH2 (C2V) MYH2 (Cs) MYH2 (Cs) MYH2 (C2V) MYH2 (Cs) MYH2 (Cs)

MYH2 (C2V) H2MY (C2V) H2MY (C2V) HMYH (Cs) HMYH (Cs) HMYH (Cs) HMYH (Cs) HMYH (Cs) HMYH (Cs)

174

[HMYH]2 (C2V) [HMYH]2 (C2V) [HMYH]2 (C2V) [HMYH]2 (D2h) [HMYH]2 (C2V) [HMYH]2 (C2V) [MYH2]2 (C2V) [HMYH]2 (C2V) [HMYH]2 (C2V)

[H2YMMYH2] (D2h) [H2MY]2 (C2V) [H2MY]2 (C2V) [MYH2]2 (C2V) [H2MY]2 (D2h) [H2MY]2 (D2h) [H2YMMYH2] (C2h) [MYH2]2 (C2V) [MYH2]2 (C2V)

276

ring [HMYH]4 (D4h) ring [HMYH]4 (D2d) [H2MY]4 (D2h) [HMYH]4 (Td) [HMYH]4 (Td) [HMYH]4 (Td) [HMYH]4 (Td) [HMYH]4 (Td) ring [HMYH]4 (D2d)

ladder [HMYH]4 (Ci) [H2MY]4 (D2h) ring [HMYH]4 (D2d) ladder [HMYH]4 (Ci) ladder [HMYH]4 (Ci) ladder [HMYH]4 (Ci) [MYH2]4 (C2h) ring [HMYH]4 (D2d) [HMYH]4 (Td)

227

212 41 183 48 24 211 74 53

Relative energy, E, in kilojoules per mole, of the second lowest isomer.

85 12 120 107 68 58 30 66

3 154 197 95 62 97 30 7



Figure 6. Optimized structures for [H2MY]4 rings. (a) [H2BN]4, (b) [H2BP]4, (c) [H2BAs]4, (d) [H2AlN]4, (e) [H2GaN]4. All distances in angstroms, angles in degrees. B3LYP/TZVP level of theory.

Figure 7. Optimized structures for bridged [H2MY]4 rings. (a) [H2BN]4, (b) [H2BP]4, (c) [H2BAs]4, (d) [H2AlN]4, (e) [H2AlP]4, (f) [H2AlAs]4, (g) [H2GaN]4, (h) [H2GaP]4, (i) [H2GaAs]4. All distances in angstroms, angles in degrees. B3LYP/TZVP level of theory.

Figure 11 in ref 10 for details of structures of dimers.) For Y ) P, As, the D2h symmetric MH2 bridged [H2MY]4 rings with planar Y4 core are true minima on their PES, whereas for Y ) N, such structures are transition states. Lowering the symmetry yielded slightly distorted structures (C2 point group, Figure

7a,d,g). However, nitrogen-containing C2 symmetric [H2MN]4 compounds are only metastable such that their dissociation into rhombic [H2MN]2 dimers is exothermic by -18, -191, and -169 kJ mol-1 for M ) B, Al, and Ga, respectively. In contrast, the P and As tetramers are stable with respect to dissociation.

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Note that group 13 elements achieve the favorable coordination number four in MH2 bridged ring compounds. As a result, such compounds are significantly stabilized and become competitive in energy with other isomers. Therefore, [H2BP]4 is only 3 kJ mol-1 higher in energy than the lowest-lying ring-[HBPH]4 isomer, and [H2BAs]4 is predicted to be the most stable isomer among all B4AS4H8 structures considered. Both [H2BP]4 and [H2BAs]4 compounds have relatively large (315-325 kJ mol-1) dissociation energies with respect to [H2MY]2 dimers (Table 3) and expected to be viable synthetic targets. In contrast with the boron-containing species, Al- and Ga-containing [H2MY]4 analogs are higher in energy with respect to cubane-type [HMYH]4 structures and have considerably smaller (150-205 kJ mol-1) dissociation energies (Table 3). 4. Conclusions A series of group 13-15 compounds of the general formula M4Y4H8 (M ) B, Al, Ga; Y ) N, P, As) has been theoretically studied at the B3LYP/TZVP level of theory. A large number of structural isomers, featuring three different bonding patterns, has been considered. Optimized structures and relative and dissociation energies were obtained. Relative energies of tetrameric compounds considered in the present work are summarized in Table 1. Note that depending on the 13-15 elements, the lowest-energy isomers adopt different bonding patterns. For B4N4H8, the optimum structure is a planar D4h symmetric ring [HBNH]4, whereas for B4P4H8, it is a D2d symmetric ring [HBPH]4, and for B4As4H8, it is the D2h symmetric ring [H2BAs]4 featuring a planar As4 core bridged by BH2 groups. For Ga4As4H8, most stable is the D2d symmetric ring of the [HGaAsH]4 bonding pattern. For the remaining 13-15 pairs, Td symmetric cubane-type [HMYH]4 structures are the most stable isomers. Different bonding patterns result in different trends in dissociation energies with respect to group 13 and group 15 elements. Therefore, whereas among isomers with the tetrahedral M4 core, boron-containing species are most strongly bound, they are the weakest bound among [HMYH]4 cubanes. Preferential bonding patterns for monomeric, dimeric, and tetrameric compounds are summarized in Table 4. From the results in Table 4, it is concluded that many structures are competitive energetically. As can be seen, there is obviously an interplay between the different types of bonding patterns, which affect the relative stabilities of the different isomers. For the (BAsH2)n series, with increasing n, structures featuring the H2BAs bonding pattern are stabilized and become the most stable isomers for n ) 4. The MYH2 bonding pattern, which was the most stable for the monomeric Al and Ga compounds, is much less observed for the dimers and becomes the least stable for the tetrameric compounds. New structural types, such as bridged ring structures featuring perfectly planar M4 and Y4 rings, have been established. In particular, the existence of the relatively stable BH2 bridged ring structures, featuring planar P4 and As4 cores, has been predicted for the first time. We hope that our theoretical predictions will stimulate further experimental efforts in the synthetic chemistry of group 13-15 elements. Acknowledgment. A.Y.T. is grateful to the Alexander von Humboldt Foundation for partial financial support of this work. Work at the University of Georgia was supported by the NSF grant CHE-0749868. Supporting Information Available: Complete ref 43 citation; optimized geometries of C4V and C2 symmetric [HMYH]4

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