Precursor Synthesis

With a view to synthesising a single-source InAs precursor, trimethylindium was reacted with a slight excess of tert-butylarsine in toluene, according to equation 4.5.

Tert-butylarsine was chosen as an arsenic source owing to its wide use in the deposition of arsenide materials and low pyrolysis temperatures.213,225,226

Both molecular dynamics studies and partial pressure analysis suggest a combination of free radical and β-elimination mechanisms in the decomposition process.^226,227

The initial reaction was carried out at -78 °C, before being allowed to warm slowly to room temperature. Upon warming, the evolution of gas was observed, presumed to be the release of methane upon formation of the product. The reaction was gently warmed to 80 °C and allowed to stir at this temperature for 16 hours, during which time gas was evolved more readily. After 16 hours gas evolution ceased and the reaction mixture was cooled to room temperature, filtered and the solvent was removed from the filtrate in vacuo.

(4.5) The ¹H and ¹³C{¹H} NMR spectra of the crude product showed a large number of resonances in the InMe, As^tBu and AsH regions, suggesting either the formation of a large oligomeric species, or probably the formation of a number of different species.

Resonances in the ¹H NMR spectrum in the region 0.42 - 0.86 ppm were attributed to InMe, those in region 1.31 - 1.62 ppm to As^tBu and two peaks at 2.51 and 2.52 ppm to AsH. Likewise, the ¹³C{¹H} NMR spectrum showed resonances in the region -5.2 - -2.0 ppm (InMe) and 31.3 – 40.4 ppm (As^tBu). These resonances are all in agreement with NMR data for related InAs clusters.^{198, 206}

The product was re-dissolved in minimal toluene and cooled to -20 °C for a number of days, after which time small, yellow, rod-like crystals formed, which were suitable for single crystal X-ray diffraction. The analysis confirmed the formation of

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the cluster shown in Figure 4.3 (hydrogen atoms omitted for clarity). Crystal data are given in Table 4.2 and selected bond lengths and angles are given in Table 4.3.

The centrosymmetric InAs cluster crystallised into the monoclinic space group P2₁/c.

Figure 4.2 shows a simplified drawing of the complex with alkyl and hydride groups omitted for clarity. The structure can be considered as two conjoined units containing eight In/As centres (Figure 4.2(a)). Each unit consists of a six-membered In-As ring in a distorted boat conformation, with each atom being four-coordinate, possessing three bonds to an adjacent As or In atom, and one further bond to either a methyl (In) or tert-butyl (As) group. Each ring is arched by a [-In(Me)2-As(^tBu)H-]

bridge. The two major units are linked by four In-As bonds between the two rings (Figure 4.2(b)) forming two additional In-As four-membered rings.

Figure 4.2 - Structural representation of the oligomeric indium arsenide precursor complex, showing a) an isolated eight-membered unit; and b) the two interconnected units, linked through In-As bonding. Alkyl

substituents have been omitted for clarity.

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Figure 4.3 - X-ray crystal structure of the indium arsenide precursor complex.

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Table 4.2 - Crystal data for indium arsenide cage complex.

Crystal system Monoclinic

Space group P21/c

Unit cell dimensions a = 14.4602(10) Å b = 12.4339(9) Å c = 23.7507(17) Å

α = 90°

β = 107.5640(10)°

γ = 90°

Each In and As centre is four-coordinate, with the indium atoms utilising their vacant p-orbitals in order to increase their coordination number from 3 to 4. Bond angles around the indium centres show a significant distortion from tetrahedral geometry, with angles ranging from 97.99° to 118.32° around In(1), which are typical for all the indium centres. The arsenic centres show a lesser degree of distortion, with typical values ranging between 104.81 and 114.08° for As(2). The In-As bond lengths were somewhat varied ranging between 2.6202(5) and 2.6836(5) Å. The In-As bonds involved in the four-member rings which link the two units are reasonably short (average 2.67 Å) for such species, by comparison to simple dimeric species [Me2InAs(SiMe3)2]2 (2.70 Å), [Et2InAs^tBu2]2 (2.70 Å) and [ⁿBu2InAs^tBu2]2 (2.72 Å), which all contain four-membered In-As rings.198, 199, 206

Whilst this might seem surprising given the anticipated increase in steric crowding for the larger oligomer in comparison to smaller dimeric structures, steric crowding is in fact less in this case due to the presence of just one alkyl substituent per In/As centre, resulting in less

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found in the monomeric species [(^tBu)2GaAs(^tBu)2], where As is only three coordinate. This is presumably a result of the removal of electron density from the As centre on becoming four-coordinate.

Table 4.3 - Selected bond lengths (Å) and angles (°) for indium arsenide cage complex [InAs]8.

As(1B)-In(1) 2.6642(10) As(2)-In(3) 2.6218(6) As(1B)-In(2B) 2.684(2) As(2)-In(4) 2.6337(5) As(3)-In(1) 2.6202(5) As(3)-In(3)#1 2.6655(5) As(3)-In(4) 2.6587(5) As(4)-In(4)#1 2.6835(5) As(4)-In(1) 2.6327(6) In(3)-As(3)#1 2.6653(5) As(4)-In(3) 2.6595(5) In(4)-As(4)#1 2.6836(5) As(2)-In(2B) 2.680(2)

C(1)-In(1) 2.175(4) C(5)-In(4) 2.179(5)

C(2B)-In(2B) 2.178(10) C(6B)-As(1B) 2.036(11) C(3B)-In(2B) 2.176(8) C(14)-As(3) 2.023(5)

C(1)-In(1)-As(3) 118.32(13) As(3)-In(1)-As(4) 102.871(16) C(1)-In(1)-As(4) 118.01(14) As(3)-In(1)-As(1B) 97.99(5) C(1)-In(1)-As(1B) 109.54(13) As(4)-In(1)-As(1B) 107.93(6)

C(10)-As(2)-In(3) 107.13(14) In(3)-As(2)-In(4) 114.079(17) C(10)-As(2)-In(4) 107.66(12) In(3)-As(2)-In(2B) 104.81(9) C(10)-As(2)-In(2B) 108.76(13) In(4)-As(2)-In(2B) 114.11(7)

Whilst this particular oligomer has been isolated and structurally characterised, it is unlikely that it is the only oligomer formed, only that this particular oligomer has preferentially crystallised. Indeed, in solution it is likely that a number of oligomers exist in equilibrium, as would be suggested by the large number of peaks present in the NMR spectra. Integration of peaks in the InMe, As^tBu and AsH regions of the ¹H NMR spectrum were in a roughly 1:36:15 ratio, which is consistent with the formation of the above complex. Furthermore, elemental analysis showed carbon to be present at 24.24% and hydrogen at 5.06%, in good agreement with the calculated values of 23.71 and 4.93%, respectively. Mass spectrometry did not show the

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presence of the parent ion for the cluster, displaying instead a large degree of fragmentation. The only peak of significant intensity was observed at 145 m/z, corresponding to [InMe₂]. Despite the uncertainty over the exact nature of the precursor, it is clear that the formation of a number of cluster complexes occurs during the synthetic procedure, containing a large number of In-As linkages and an In to As ratio of 1. This presents the precursor as potentially suitable for the formation of stoichiometric InAs via thermal decomposition.

Attempts to isolate monomeric species by addition of the Lewis base 4-(dimethylamino)pyridine (dmap) were carried out by addition of dmap to both the reaction mixture and the isolated product, a method well utilised by Schulz et al. in the isolation of monomeric III/V compounds.²³⁰ However, these attempts were unsuccessful with analysis continuing to show a complex mixture of products.