Novel carbon dioxide and carbonyl carbonate complexes of molybdenum. The X-ray structures of trans-[Mo(CO₂)₂{HN(CH₂CH₂PMe₂)₂}(PMe₃)] and [Mo₃(μ₂-CO₃)(μ₂-O)₂(O)₂(CO)₂(H₂O)(PMe₃)₆]·H₂O

Abstract

A new bis(carbon dioxide) adduct of molybdenum containing the tridentate, bis(phosphine) ligand, HN(CH₂CH₂PMe₂)₂ (NP₂), in a mer conformation has been synthesized and structurally characterized. Its carbonyl carbonate isomer [Mo(CO₃)(CO)(NP₂)(PMe₃)] has also been prepared. In addition, the reactivity of the complex [Mo(CO₃)(CO)(PMe₃)₄] toward CO has been studied, a transformation that has led to the formation of the dicarbonyl complex [Mo(CO₃)(CO)₂(PMe₃)₃]. This complex liberates CO₂ upon heating under a carbon monoxide atmosphere by means of a CO–CO₃ conproportionation reaction. The new trinuclear complex [Mo₃(μ₂-CO₃)(μ₂-O)₂(O)₂(CO)₂(H₂O)(PMe₃)₆]·H₂O, resulting from the interaction of [Mo(CO₃)(CO)(PMe₃)₄] and water, has been isolated and structurally characterized.

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Introduction

The serious environmental problems associated with global warming have stimulated further academic and industrial research into carbon dioxide, as this substance is a major component of the greenhouse gases.¹ In addition, there is growing interest in the use of CO₂ as a C1 source for chemicals and fuels, as well as in a variety of technological applications.² A key step in the chemical, photochemical and electrochemical transformations of CO₂ is the formation of a M–CO₂ complex, since coordination to a metal centre activates this rather unreactive molecule toward a number of chemical reactions. It is noteworthy that despite the importance of the M–CO₂ functionality, the number of structurally characterized mononuclear complexes is still limited.

Of the three coordination modes that may be envisaged for a M–CO₂ unit, namely the side-on κ²-C,O (Scheme 1, structure A) and the end-on κ¹-C or κ¹-O (B and C, respectively), only A and B have been authenticated by X-ray crystallography. κ¹-C coordination has been found in [RhCl(CO₂)(diars)]^3a and Ru(CO₂)(CO)(bipy)₂]·3H₂O,^3b whereas side-on binding (A) has been demonstrated in complexes of both the early and the late transition elements.^4,5 All these compounds contain a single M–CO₂ unit, with the only exception of some Mo complexes prepared by our group some years ago, which possess a trans-Mo(CO₂)₂ functionality.⁵

Scheme 1 Coordination modes of carbon dioxide.

One of the reactions a coordinated molecule of CO₂ may undergo is reduction to CO by means of oxygen atom transfer to another substrate. The latter can be an oxophilic metal⁶ or a phosphine ligand⁷ (or other readily oxidized ligand). But it is also possible for CO₂ to act as its own oxygen sink, giving rise to CO and CO₃²⁻ [eqn. (1)] in a reaction called reductive disproportionation of CO₂:^7,8

2 CO₂ + 2 e⁻ → CO₃²⁻ + CO (1)

Most often this is an irreversible process that leads to carbonato complexes, nevertheless in a few instances, a metal-carbonate complex has been shown to react with CO to produce CO₂.⁹ This is the reverse of eqn. (1), that is the oxidative conproportionation of carbon dioxide.

Following previous work on the generation of Mo–CO₂ adducts⁵ and Mo carbonyl carbonate complexes,⁸ we would like to report new findings in this area that include the structural characterization by X-ray methods of the bis(CO₂) adduct, trans-[Mo(CO₂)₂(NP₂)(PMe₃)] (1), which contains the tridentate ligand HN(CH₂CH₂PMe₂)₂ (NP₂). An X-ray study on the trinuclear, mixed-valence Mo(II)–Mo(VI) complex of composition [Mo₃(μ₂-CO₃)(μ₂-O)₂(O)₂(CO)₂(H₂O)(PMe₃)₆]·H₂O (4) is also reported.

Results and discussion

Synthesis and molecular structure of trans-[Mo(CO₂)₂(NP₂)(PMe₃)] (1)

We have shown earlier that the Mo(0) compound trans-[Mo(CO₂)₂(PMe₃)₄] exhibits a characteristic substitution chemistry that allows stepwise replacement of the labile PMe₃ groups, without alteration of the trans-Mo(CO₂)₂ linkage.⁵ During formation of compounds of composition trans-[Mo(CO₂)₂(PMe₃)₃(CNR)], trans-[Mo(CO₂)₂(PMe₃)₂(Me₂PCH₂CH₂PMe₂)], and others,^5b the two CO₂ ligands remain staggered with respect to one another and eclipse the trans-L–Mo–L′ vectors of the plane perpendicular to the trans-Mo(CO₂)₂ bond axis. This conformation, found to be the most favourable by ab initio calculations with the model compound trans-[Mo(CO₂)₂(PH₃)₄], maximizes the Mo(CO₂)₂ bonding interaction.¹⁰

As previous substitution chemistry involved either mono- or bidentate ligands,⁵ we wondered if the use of a tridentate ligand could force the two molecules of CO₂ into a cis geometry, and perhaps induce their subsequent coupling to give the corresponding head-to-tail or head-to-head dimers.¹¹ Edwards’ HN(CH₂CH₂PMe₂)₂ (NP₂) was synthesized¹² and used for this purpose. Nonetheless, and despite the flexibility of this and related ligands that allows them to coordinate to the metal either in facial or meridional forms,¹³ the addition of NP₂ to solutions of trans-[Mo(CO₂)₂(PMe₃)₄] gives the trans-Mo(CO₂)₂ compound 1 [eqn. (2)] as the only detectable product:

Monitoring of the reaction by ³¹P{¹H} NMR spectroscopy shows exclusive appearance of signals due to an AMX spin system and to free PMe₃. From the resulting solutions, compound 1 can be isolated as a yellow crystalline material in ca. 70% yield. 1 is soluble in aromatic hydrocarbons, tetrahydrofuran, methanol and other common aromatic solvents. Solutions in methanol exhibit an orange colour, perhaps due to extensive hydrogen bonding involving molecules of the solvent.

The IR spectrum of 1 shows bands at 1660, 1155 and 1100 cm⁻¹, which can be assigned to vibrations arising from the coordinated molecules of CO₂ by comparison with the spectrum of 1* (ca. 30% ¹³CO₂-enriched). These absorptions have strikingly similar energies to those of the starting material and of other trans-Mo(CO₂)₂ adducts.⁵ In the ¹H NMR spectrum, inequivalency of the P–Me groups of the tridentate ligand is evidenced by the observation of four independent doublets in the range δ 0.9–1.4, whereas the molecule of PMe₃ gives a doublet centred at δ 1.33 (J_P-H = 8.7 Hz). As already indicated, resonances attributable to an AMX spin system are discerned in the ³¹P{¹H} NMR spectrum. Those due to the ³¹P termini of the NP₂ ligand (A and X) show the effect of a strong J(P_A-P_X) coupling of 178 Hz, indicative of a mutual trans geometry. The single PMe₃ group (P_M), cis with respect to P_A and P_X, appears as a triplet due to accidentally degenerate P_A-P_M and P_A-P_X couplings of 12 Hz. The room temperature ¹³C{¹H} NMR spectrum of 1* consists of two broad, partially unresolved signals centred at about δ 213.5 and 217, suggestive of fluxionality. Accordingly, upon cooling to −80 °C, two well-resolved signals, each consisting of a doublet of doublets of doublets, are observed (see Experimental). The fluxionality of related trans-Mo(CO₂)₂ adducts has been studied in detail and demonstrated to consist of a synchronous motion of the two CO₂ ligands in which both molecules rotate in the same direction.^5b

The spectroscopic data discussed in the preceding paragraph supports the geometry proposed for complex 1 in eqn. (2). Unequivocal confirmation has been provided by single-crystal X-ray studies, whose results are presented in Fig. 1 and Tables 1 and 2. Compound 1 has a distorted octahedral geometry, with the CO₂ ligands side-on bonded to molybdenum through one of the C=O bonds. In agreement with theory,¹⁰ and with structures of analogous derivatives,^5a the two CO₂ ligands are mutually staggered, each one eclipsing a corresponding trans-L–Mo–L′ vector in the plane perpendicular to the CO₂–Mo–CO₂ axis. Bonding parameters within the Mo–CO₂ units are similar to those found for trans-Mo(CO₂)₂(PMe₃)₃(CN–i-Pr) and reveal, likewise, strong Mo–CO₂ bonding interactions. The Mo–C bond lengths in 1 (2.08 Å ave) are identical within experimental error to those in the above CN–i-Pr derivative and approach normal Mo–CO distances {1.970(4) and 2.03(1) Å, average values for the two types of CO groups in cis-[Mo(CO)₄(PMe₃)₂]¹⁴}. The same argument applies to the Mo–O bonds in 1 (2.12 Å ave). The two C–O bonds of each molecule of CO₂ appear to be of identical length within experimental error (Table 2) but are longer than in free CO₂ (1.16 Å).

Fig. 1 ORTEP view of the molecule in compound 1.

Table 1 Crystal data and structure refinement for compounds 1 and 4

	1	4
Empirical formula	C13H30MoNO4P3 (C3H8 )0.5	C21H56MoO10P6·H2O
Formula weight	474.25	960.3
Crystal system	Monoclinic	Monoclinic
Space group	C2/c	P21/n
a/Å	19.947(5)	9.769(1)
b/Å	11.670(2)	16.333(1)
c/Å	20.004(7)	24.682(1)
β/°	101.89(2)	98.99(1)
U/Å^3	4557 (2)	3889.9(5)
Z	8	4
T/K	295	294
μ/mm^−1	0.801	1.244
Reflexions measured	5616	11 422
Unique reflexions	5459	11 177
R 1 (all data)	0.236	0.0760
wR 2 (all data)	0.238	0.1038
R 1 [I > 2σ(I)]	0.064	0.0327
wR 2 [I > 2σ(I)]	0.173	0.0889

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Table 2 Main bond distances (Å) and angles (°) for compound 1^a

a Esd are given in parentheses
Mo–Pl	2.434(3)	C24–N1	1.45(2)
Mo–P2	2.481(3)	C34–N1	1.46(2)
Mo–P3	2.443(3)	C1–O1	1.284(11)
Mo–N1	2.270(7)	Cl–O2	1.199(10)
Mo–C1	2.088(9)	C2–O3	1.245(11)
Mo–O1	2.111(6)	C2–O4	1.234(11)
Mo–C2	2.085(9)	Hl–N1	0.91(11)
Mo–O3	2.138(6)
C1–Mo–C2	148.6(4)	C1–Mo–P2	92.7(3)
C1–Mo–O1	35.6(3)	C2–Mo–P2	113.0(3)
C2–Mo–O1	157.1(4)	O1–Mo–P2	84.1(2)
C1–Mo–O3	156.5(3)	O3–Mo–P2	78.9(2)
C2–Mo–O3	34.3(3)	N1–Mo–P2	79.5(2)
O1–Mo–O3	158.9(3)	P1–Mo–P2	97.8(1)
C1–Mo–N1	116.2(4)	P3–Mo–P2	156.0(1)
C2–Mo–N1	87.2(3)	C24–N1–C34	110.0(8)
O1–Mo–N1	80.7(3)	C24–N1–Mo	113.7(6)
O3–Mo–N1	84.0(4)	C34–N1–Mo	115.0(6)
C1–Mo–P1	76.2(3)	C24–N1–H1	105.8
C2–Mo–P1	82.3(2)	C34–N1–H1	105.8
O1–Mo–P1	111.6(2)	Mo–N1–H1	105.8
O3–Mo–P1	83.2(2)	O2–C1–O1	131.2(9)
N1–Mo–P1	167.2(2)	O2–C1–Mo	155.6(8)
C1–Mo–P3	84.9(3)	O1–C1–Mo	73.2(5)
C2–Mo–P3	78.6(3)	C1–O1–Mo	71.2(5)
O1–Mo–P3	80.2(2)	O4–C2–O3	131.6(10)
O3–Mo–P3	111.5(2)	O4–C2–Mo	153.2(9)
N1–Mo–P3	80.1(2)	O3–C2–Mo	75.2(5)
P1–Mo–P3	104.8(1)	C2–O3–Mo	70.5(5)

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The rigidity of the Mo(NP₂) chelate causes significant deviations of the corresponding bond angles from the ideal 90° and 180° values expected for an octahedral geometry. For instance, the two N–Mo–P angles approach 80°, whereas that between the two, supposedly trans, Mo–P bonds amounts to only 155.9(1)°. Considering this, the mer arrangement of the NP₂ ligand, although not unprecedented,¹³ may seem unexpected. It appears that the flexibility of the N–CH₂–CH₂–P arms allow the tetrahedral nitrogen donor to be accommodated in a mer structure [angles centred on N: C24–N–Mo, 113.7(6)°; C34–N–Mo, 115.0(6)°; C24–N–C34, 110.0(8)°], in this way keeping the trans-Mo(CO₂)₂ moiety unaltered. The Mo–N and Mo–P separations have normal values (Table 2). An additional interesting feature of the solid state structure of 1 is the intermolecular hydrogen bond interaction that involves the aminic HNP₂ hydrogen and an exo (i.e., non-coordinated) oxygen atom of a neighbouring molecule (Fig. 2). The observed H1–O_exo distance of 2.2 Å (ave) is shorter that the sum of the van der Waals’ radii¹⁵ [1.40 (O) + 1.2 (H) = 2.60 Å], clearly in support of this proposal. Protic solvents like methanol (vide supra) may also participate in hydrogen bonding. An extended 3D network of hydrogen bonds has been shown to exist in the solid state structure of the η¹-CO₂ adduct [Ru(CO₂)(CO)(bipy)₂]·3H₂O.^3b

Fig. 2 Molecular view of two molecules of 1 showing the hydrogen bonds.

Carbonyl carbonate complexes of Mo. Molecular structure of [Mo₃(μ₂-CO₃)(μ₂-O)₂(O)₂(CO)₂(H₂O)(PMe₃)₆]·H₂O (4).

In coordinating solvents, cis-[Mo(N₂)₂(PMe₃)₄] induces the reductive disproportionation of CO₂ with formation of the carbonyl carbonates [Mo(CO₃)(CO)(PMe₃)₄] and [Mo(CO₃)(CO)(PMe₃)₃]₂. Both complexes interconvert readily by loss or addition of PMe₃, and react with chelating diphosphine ligands (P-P) to afford the substitution products [Mo(CO₃)(CO)(P-P)(PMe₃)₂] and [Mo(CO₃)(CO)(P-P)₂].^8b

In a like manner, addition of NP₂ to solutions of either of the above carbonato compounds allows the isolation of a yellow microcrystalline species [eqn. (3)] of composition [Mo(CO₃)(CO)(NP₂)(PMe₃)] (2):

The presence of the NP₂ ligand in the molecules of 2 is indicated by the observation of an IR ν(N–H) band at 3300 cm⁻¹ and by its characteristic NMR parameters, detailed in the Experimental. On account of the appearance of an AX₂ spin system for the ³¹P nuclei of 2 (δ_A = 24.4, δ_X = 69.1; ²J_AX = 19 Hz), the NP₂ ligand is suggested to adopt a mer conformation, as demonstrated by X-ray studies for 1. A distinctive IR absorption at 1600 cm⁻¹ can be taken as diagnostic of bidentate, κ²-O,O′ coordination of the carbonate group.¹⁶ This is not shifted in comparison with the starting material, whilst ν(CO) for the Mo–CO unit moves from 1810 cm⁻¹ in [Mo(CO₃)(CO)(PMe₃)₄] to 1780 cm⁻¹. It thus appears that replacement of three PMe₃ groups by NP₂ increases back-donation from Mo to CO. On the basis of the above data compound 2 can be considered structurally comparable to the parent carbonate [Mo(CO₃)(CO)(PMe₃)₄].^8b

At variance with this result, only one PMe₃ ligand is substituted by CO when [Mo(CO₃)(CO)(PMe₃)₄] is reacted at room temperature, with 4 atm of carbon monoxide [eqn. (4)]:

The ³¹P{¹H} NMR spectrum of 3 recorded at 20 °C displays two broad resonances that convert into well-resolved triplet and doublet upon cooling to −80 °C [AX₂ spin system, δ_A −8.7, δ_X 35.1; J(P_A-P_X) = 11.7 Hz]. The well-defined band in the vicinity of 1600 cm⁻¹ due to the ν(C=O) of the carbonate shifts slightly to lower energy (1580 cm⁻¹), while the two CO groups are at the origin of absorptions at 1920 and 1820 cm⁻¹.

Somewhat unexpectedly, reduction to Mo(0) occurs upon mild heating (60 °C) of solutions of carbonato 3. The reaction becomes cleaner when performed in the presence of CO; a mixture of the known carbonyl derivatives [Mo(CO)_n(PMe₃)_6 − n] (n = 2, 3)¹⁷ is obtained under these conditions. Analysis of the volatiles reveal the presence of CO₂ further identified by its known, characteristic reaction^9a,b with the complex , in the presence of small amounts of water, to give the carbonate [Ni₂(CH₂CMe₂Ph)₂(μ-CO₃)(PMe₃)₃]. These observations indicate that, upon heating, the CO₃²⁻ and CO ligands of 3 undergo an oxidative conproportionation to CO₂ [eqn. (5)], with concomitant reduction of the metal from Mo(II) to Mo(0):

CO₃²⁻ + CO − 2 e⁻ → 2 CO₂ (5)

Besides [Mo(CO₃)(CO)(PMe₃)₄] and [Mo(CO₃)(CO)(PMe₃)₃]₂, the reaction of cis-[Mo(N₂)₂(PMe₃)₄] and CO₂ provides, under certain conditions, small amounts of another carbonato derivative, namely the known^8b tetranuclear [Mo₄(CO₃)(μ-O)₂(μ-OH)₄(CO)₂(PMe₃)₆]. As depicted schematically in D and E, this compound contains a unique bridging carbonate group. Heating a THF solution of [Mo(CO₃)(CO)(PMe₃)₃]₂ plus added water, at 50 °C, gives the same Mo₄ species, albeit once more in low yields (ca. 10%).

In an attempt to find a reliable, higher-yield synthetic procedure, [Mo(CO₃)(CO)(PMe₃)₄] and [Mo(CO₃)(CO)(PMe₃)₃]₂ have been reacted with H₂O under different experimental conditions, including in the presence of air. Acetone, methanol and tetrahydrofuran, as well as some of their mixtures, have been tested with controlled addition of H₂O at room temperature and at 50–60 °C. A sufficiently better route to the Mo₄ complex has not been found but nevertheless a somewhat related Mo₃ complex of composition [Mo₃(μ₂-CO₃)(μ₂-O)₂(O)₂(CO)₂(H₂O)(PMe₃)₆]·H₂O (4) has been isolated. Due to its insolubility in common organic solvents, reliable NMR data have not been recorded. Its IR spectrum shows a band at ca. 3500 cm⁻¹ due to a coordinated molecule of water, along with carbonyl absorptions at 1770 and 1755 cm⁻¹ and others attributable to the carbonate (1515 and 1280 cm⁻¹) and PMe₃ (945 cm⁻¹) ligands. The structure of the molecule of 4 has been unambiguously determined by a single-crystal X-ray study, whose results are presented in Fig. 3 and Tables 1 and 3.

Fig. 3 ORTEP view of the molecule of compound 4.

Table 3 Main bond distances (Å) and angles (°) for compound 4^a

a Esd are given in parentheses
Mo1–Pl	2.437(1)	Mo2–O6	2.189(2)
Mo1–P3	2.438(1)	Mo2–C1	2.598(3)
Mo1–P5	2.452(1)	Mo2–C3	1.918(4)
Mo1–O1	2.232(2)	Mo3–O5	1.749(3)
Mo1–O8	2.167(2)	Mo3–O6	1.773(2)
Mo1–O10	2.271(2)	Mo3–O7	1.739(3)
Mo1–C2	1.887(4)	Mo3–O8	1.783(2)
Mo2–P2	2.412(1)	O1–C1	1.265(4)
Mo2–P4	2.424(1)	O2–C1	1.284(4)
Mo2–P6	2.442(1)	O3–C1	1.304(4)
Mo2–O2	2.202(2)	O4–C2	1.188(4)
Mo2–O3	2.204(2)	O9–C3	1.181(4)
O10–Mo1–C2	130.1(1)	P6–Mo2–C3	76.7(1)
O8–Mo1–C2	131.5(1)	P6–Mo2–O6	158.75(7)
O8–Mo1–O10	81.98(9)	P6–Mo2–O3	79.45(6)
O1–Mo1–C2	127.3(1)	P6–Mo2–O2	82.53(6)
O1–Mo1–O10	86.35(8)	P4–Mo2–C3	75.2(1)
O1–Mo1–O8	82.81(8)	P4–Mo2–O6	76.50(7)
P5–Mo1–C2	73.6(1)	P4–Mo2–O3	90.10(6)
P5–Mo1–O10	79.43(7)	P4–Mo2–O2	145.35(6)
P5–Mo1–O8	154.94(7)	P4–Mo2–P6	109.66(4)
P5–Mo1–O1	79.4(6)	P2–Mo2–C3	73.0(1)
P3–Mo1–C2	74.5(1)	P2–Mo2–O6	79.27(6)
P3–Mo1–O10	79.99(6)	P2–Mo2–O3	140.30(6)
P3–Mo1–O8	78.04(6)	P2–Mo2–O2	84.13(6)
P3–Mo1–O1	157.81(6)	P2–Mo2–P6	113.30(4)
P3–Mo1–P5	114.70(3)	P2–Mo2–P4	117.31(3)
P1–Mo1–C2	74.3(1)	O7–Mo3–O8	109.5(1)
P1–Mo1–O10	155.59(7)	O6–Mo3–O8	110.9(1)
P1–Mo1–O8	79.98(6)	O6–Mo3–O7	108.0(1)
P1–Mo1–Ol	75.22(6)	O5–Mo3–O8	111.1(1)
P1–Mo1–P5	111.92(3)	O5–Mo3–O7	108.9(1)
P1–Mo1–P3	111.89(3)	O5–Mo3–O6	108.3(1)
O6–Mo2–C3	124.4(1)	O2–C1–O3	115.3(3)
O3–Mo2–C3	145.5(1)	O1–C1–O3	122.1(3)
O3–Mo2–O6	80.24(8)	O1–C1–O2	122.7(3)
O2–Mo2–C3	139.4(1)	Mo1–C2–O4	179.2(3)
O2–Mo2–O6	81.84(9)	Mo2–C3–O9	179.1(3)
O2–Mo2–O3	59.49(8)

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As can be seen, complex 4 is a mixed-valence Mo(II)–Mo(VI) trinuclear species that consists of two seven-coordinate Mo(II) atoms bridged by a κ²-O,O′-MoO₄²⁻ unit. The structural relation of the moiety comprised by the two Mo(II) atoms with [Mo(CO₃)(CO)(PMe₃)₃]₂ and the structurally characterized PMe₂Ph analogue is evident.^7a One of the μ-CO₃²⁻ ligands of [Mo(CO₃)(CO)(PMe₃)₃]₂ is lost, being formally replaced by the molybdate group. As the remaining CO₃²⁻ acts as a κ²-O,O’ toward Mo(2) and as κ¹-O” toward Mo(1), the coordination of the latter is completed by a molecule of H₂O. Note that the “Mo(CO)(PMe₃)₃” terminal fragments are maintained both in 4 and in the Mo₄ complex referred to above. In complex 4, the CO₃²⁻ group that bridges these fragments is planar: the three O–C–O angles have similar values that add together to practically 360°. The three C–O bonds do not differ appreciably in length and the same can be said for the three Mo–OCO₂ separations.

Despite its bidentate nature, the bridging MoO₄²⁻ group has a nearly tetrahedral geometry, with O6–Mo3–O7 [108.0(1)°] and O5–Mo3–O8 [111.1(1)°] showing the largest deviations with respect to the 109.5° ideal value. The two Mo–O_t bonds (Mo3–O5 and Mo3–O7) are identical within experimental error and average 1.74 Å. The bridging Mo–O bonds are also of equal length (1.77 Å, average value) and only slightly longer than the Mo–O_t. For comparative purposes, the Mo–O distance in K₂MoO₄ is 1.76(1) Å.¹⁸ Trimetallic units are bonded among themselves, via intermolecular hydrogen bonds (Table 4) through the coordinated water molecules in the [010] direction and through the uncoordinated ones in the [100] direction. This arrangement gives rise to infinite layers perpendicular to the [001] direction of composition {[Mo₃(CO)₂(PMe₃)₆(H₂O)(μ₂-CO₃)(O)₂(μ₂-O₂)]H₂O}^_∞. The only two oxygen atoms that are not involved in the connectivity inside these layers, those of the carbonyl groups O(4) and O(9), are pointing outside, toward the interlayer space (Fig. 4).

Fig. 4 View of the packing showing the connections among the Mo trimetallic units of 4 through the coordinated and uncoordinated water molecules in the b (left) and a (right) directions, respectively. Blue circles are terminal oxygen atoms (O4 and O9) and uncoordinated water oxygen (O11), grey circles represent the carbon atoms of the carbonate (C1).

Table 4 Hydrogen bond distances (Å) and angles (°) for 4

O–H⋯O^a	d(O–H)	d(H⋯O)	d(O⋯O)	∠(O–H⋯O)
a Esd are given in parentheses. Symmetry code: (′) 3/2 − x, y − 1/2, 1/2 − z. (′′) x − 1, y, z.
O10–H101⋯O5′	0.77(6)	1.94(6)	2.683(4)	164(5)
O10–H102⋯O3	0.96(6)	1.74(6)	2.599(4)	148(5)
O11–H111⋯O7′′	0.92(6)	1.99(7)	2.902(6)	172(5)
O11–H112⋯O2	0.94(6)	1.98(6)	2.890(5)	160(5)

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The identification of the molybdate group in the structure of 4 allows the design of a rational preparative route, consisting in the reaction of stoichiometric amounts of [Mo(CO₃)(CO) (PMe₃)₃]₂, dissolved in THF, and of Na₂MoO₄, dissolved in water (yields are about 50%). Even though 4 may be an intermediate along the path leading to the Mo₄ compound, we have been unable to find consistent proof for this hypothesis. An alternative possibility is that they form independently. Regardless of this, their insolubility in common solvents permits their isolation and ulterior characterization.

Conclusion

In summary, the electronically favourable trans-Mo(CO₂)₂ conformation, with staggered CO₂ ligands, characteristic of the complex trans-[Mo(CO₂)₂(PMe₃)₄] is retained during the substitution of three molecules of PMe₃ by the tridentate HN(CH₂CH₂PMe₂)₂ ligand (NP₂), to give compound 1. The isomeric carbonyl carbonate complex, [Mo(CO₃)(CO)(NP₂)(PMe₃)], has also been prepared, along with an unusual trinuclear complex 4, shown by X-ray studies to consist of two seven-coordinate Mo(II) atoms bridged by a bidentate MoO₄²⁻ unit. The trimetallic units of 4 are bonded among themselves by means of intermolecular hydrogen bonds that involve both the coordinated and the uncoordinated water molecules of 4, forming the infinite layers schematically represented in Fig. 4.

During the revision of this work, a linear, O-coordinated carbon dioxide complex of uranium has been reported.¹⁹

Experimental

All preparations and manipulations were performed under an oxygen-free nitrogen atmosphere using conventional Schlenk techniques. Solvents were rigorously dried and degassed before use. The complexes trans-[Mo(CO₂)₂(PMe₃)₄],⁵ [Mo(CO₃)(CO)(PMe₃)₄],⁸ [Mo(CO₃)(CO)(PMe₃)₃]₂,⁸ and ⁹ and the phosphine HN(CH₂CH₂PMe₂)₂¹² were prepared according to literature methods. NMR spectra were acquired with Varian XL-200 and Bruker DRX 500 MHz spectrometers. Microanalyses were carried out by the Microanalytical Service of the University of Seville. Despite all our attempts, satisfactory analytical data have been obtained only for 1 and 4.

Syntheses

trans-[Mo(CO₂)₂(HN(CH₂CH₂PMe₂)₂)(PMe₃)] (1). A solution of trans-[Mo(CO₂)₂(PMe₃)₄] (0.1 g, 0.2 mmol) in tetrahydrofuran (30 ml) was treated with a slight excess of HN(CH₂CH₂PMe₂)₂ (0.048 g, 0.25 mmol). After 2 h of stirring, the volatiles were removed under reduced pressure. The residue was extracted with a 1∶1 mixture of Et₂O–petroleum ether, and the resulting solution was cooled to −20 °C, giving yellow crystals of complex 1. Yield: 70%. Anal. found: C, 34.0; H, 6.5; N, 3.1; calcd. for C₁₃H₃₀MoNO₄P₃: C, 34.5; H, 6.7; N, 3.1%. IR (cm⁻¹): 3500, 3200, ν(N–H); 1660, 1150, 1100, ν(C=O); 950, ν(P–C). NMR (see F for numbering scheme): ¹H (CD₃OD, 500 MHz) δ 3.96 [br t, N–H, J(HH₆) = 11.6, J(HH₅) = 11.6, J(HH₇) = 3.5, J(HH₈) = 3.5, J(HP_X) = 37.2 Hz ], 3.19 [m, H₈, J(H₈H₆) = 11.6, J(H₈-H₁) = 4.8, J(H₈-H₇) = 2.1, J(H₈-H₄) = 1.2 Hz], 3.14 [m, H₇, J(H₇-H₅) = 11.6, J(H₇-H₂) = 4.8, J(H₇-H₃) = 2.1, J(H₇-P_A) = 35.8 Hz], 2.58 [m, H₆, J(H₆-H₁) = 14.7, J(H₆-H₄) = 3.2, J(H₆-P_X) = 7.8 Hz ], 2.50 [m, H₅, J(H₅-H₁) = 14.8, J(H₅-H₃) = 3.2, J(H₅-P_A) = 37.2 Hz], 2.13 [br t, H₄, J(H₄-H₁) = 14.4, J(H₄-P_X) = 13.2 Hz], 1.99 [br t, H₃, J(H₃-H₂) = 12.9, J(H-P_A) = 12.9 Hz], 1.86 [m, H₂, J(H₂-P_A) = 4.1 Hz], 1.78 [d, 3H, P_A–Me, J(H-P) = 9.1 Hz], 1.66 [m, H₁, J(H₁-P_X) = 4.3 Hz], 1.39 [d, 3H, P_X–Me, J(H-P) = 7.8 Hz], ], 1.33 [d, 9H, P–Me₃, J(H-P) = 8.7 Hz], ], 1.30 [d, 3H, P_X–Me, J(H-P) = 8.3 Hz], 0.94 [d, 3H, P_A–Me, J(H-P) = 8.1 Hz]; ¹³C{¹H} (75 MHz, CH₃OH–CD₃COCD₃, −80 °C, sample 30% enriched in ¹³CO₂) δ 218.6 [ddd, 1 CO₂, J(C-P) = 49.1, J(C-P) = 15.5, J(C-P) = 7.8 Hz], 214.1 [ddd, 1 CO₂, J(C-P) = 43.7, J(C-P) = 18.5, J(C-P) = 10.5 Hz]. At 25 °C, these resonances appear as two multiplets with the same chemical shifts. ¹³C{¹H} (125 MHz, C₆D₆, 25 °C, non-enriched sample) δ 210.5 [m, 1 CO₂], 208.0 [m, 1 CO₂], 49.3 [s, CH₂], 48.0 [s, CH₂], 34.1 [d, P–CH₂, J(C-P) = 35 Hz ], 31.8 [d, P–CH₂, J(C-P) = 28 Hz ], 20.0 [d, P(CH₃)₃, J(C–P) = 40 Hz ], 12.2 [d, P_A–Me, J(C-P) = 39 Hz], 9.5 [d, P_X–Me, J(C-P) = 28.5 Hz], 9.0 [d, P_X–Me, J(C-P) = 28.1 Hz], 8.7 [d, P_A–Me, J(C-P) = 29.1 Hz]; ³¹P{¹H} (C₆D₆, 121 MHz), AMX spin system, δ_A 28.3 [P_A, J(P_A-P_M) = 12, J(P_A-P_X) = 178 Hz], δ_M 22.9 [PMe₃, J(P_M-P_X) = 12 Hz], δ_X 15.0 [P_X].

[Mo(CO₃)(CO)(HN(CH₂CH₂PMe₂)₂)(PMe₃)] (2). To a stirred solution of [Mo(CO₃)(CO)(PMe₃)₄] (0.1 g, 0.2 mmol), an equimolar amount of HN(CH₂CH₂PMe₂)₂ was added (0.024 g, 0.125 mmol). The addition induced an immediate change in colour from the initial red to orange, whereupon the mixture was stirred for 1 h and the volatiles were then removed. The residue was washed with Et₂O (2 × 20 ml) to give complex 2 as a yellow microcrystalline material in 60% yield. IR (cm⁻¹): 3300, ν(N–H); 1750, ν(CO); 1600, 1250, ν(CO₃); 950, ν(P–C). ³¹P{¹H} (THF–CD₃COCD₃, 81 MHz), AX₂ spin system, δ_A 24.5, δ_X 69.1 [J(P_A-P_X) = 19 Hz]; ¹H (CD₃OD, 200 MHz) 1.65 [d, 6H, 2 Me, J(H-P) = 10.2 Hz], 1.45 [d, 6H, 2 Me, J(H-P) = 9.8 Hz], 1.37 [d, 9H, PMe₃, J(H-P) = 8.6 Hz], the methylene signals appeared as broad multiplets centred at 4.55, 3.22, 2.64 and 2.00 ppm (2 H each).

[Mo(CO₃)(CO)₂(PMe₃)₃] (3). A solution of [Mo(CO₃)(CO)(PMe₃)₄] (0.1 g, 0.2 mmol) in methanol (3 ml) was pressurised with CO (4 atm) and the mixture was stirred for 2 h at room temperature. The initially red solution turned to orange; the solvent was then removed under vacuum and the residue washed with Et₂O (2 × 20 ml). Yellow microcrystalline material of complex 3 was obtained in 65% yield. Selected spectroscopic data: IR (cm⁻¹) 1920, 1820, ν(CO); 1580, ν(CO₃); 955, ν(P–C). ³¹P{¹H} (THF–CD₃COCD₃, 81 MHz) 32.5 (br s, 2 P), −10.5 (br s, 1 P). These resonances resolved into an AX₂ spin system when recording the spectrum at −80 °C: δ_A −8.7, δ_X 35.1 [J(P_A-P_X) = 11.7 Hz]. The proton and carbon spectra gave broad signals with no useful information being gained from them.

Heating a methanolic solution of complex 3 at 60 °C under CO (1 atm) for 3 h results in a change of colour from the initial orange to dark yellow. The reaction gases are trapped inside of a liquid nitrogen-cooled ampoule, a white solid (CO₂) being obtained. The former solution was investigated by NMR, a mixture of carbonyls of composition [Mo(CO)₆(PMe₃)_6 − n] being detected. The trapped gas was transferred into a flask containing a wet solution of the nickelacycle compound and the mixture stirred for 2 h, before volatile removal and NMR investigation of the crude sample.

[Mo₃(CO)₂(PMe₃)₆(H₂O)(μ₂-CO₃)(O)₂(μ₂-O)₂] (4). [Mo(CO₃)(CO)(PMe₃)₄] (0.2 g, 0.4 mmol) was suspended in acetone (8 ml) and water was added dropwise until dissolution was achieved. The red-orange solution was left undisturbed for a week. Crystals (25% yield) were then collected by filtration, the material being too insoluble to perform NMR studies. Some of the crystals were suitable for an X-ray diffraction study (see below) that led to the characterization as the title compound. Anal. found: C, 26.0; H, 5.6; calcd. for C₂₁H₅₆Mo₃O₁₀P₆·H₂O: C, 26.2; H, 6.0%. IR (cm⁻¹) 1770, 1755, ν(CO); 1515, 1280, ν(CO₃); 945, ν(P–C).

An alternative procedure has been developed. A solution of Na₂MoO₄ in water (5 ml) was transferred into a flask containing [Mo(CO₃)(CO)(PMe₃)₄] (0.2 g, 0.4 mmol) suspended in acetone (10 ml). Upon addition, the starting material dissolved at the same time that a new orange-coloured solid came out of the solution. The mixture was stirred overnight, the solid filtered off, washed with THF and Et₂O, and dried under vacuum. The resulting orange solid displayed an IR spectrum identical to that described above. Yield: 50%.

Crystal structure determinations for compounds 1 and 4†

A summary of the fundamental crystal data is given in Table 1. In both cases, a crystal of prismatic shape was coated with resin epoxy and mounted in a Kappa diffractometer. Data were collected using graphite monochromated Mo-Kα (λ = 0.71069 Å) radiation. The cell dimensions were refined by least-squares fitting of the θ values of 25 reflections. The intensities were corrected for Lorentz and polarization effects. Scattering factors for neutral atoms and anomalous dispersion corrections for Mo and P were taken from the International Tables for X-ray Crystallography.²⁰ The structure was solved by Patterson and Fourier methods. An empirical absorption correction was applied at the end of the isotropic refinement. A final refinement was undertaken with anisotropic thermal parameters for the non-hydrogen atoms. In the case of complex 1, the H1 atom was located in a difference synthesis and its coordinates were refined. Final difference synthesis showed three peaks that were assigned to 3-carbon chains originating from the petroleum ether used as the solvent. Most of the calculations were carried out with the SHELXTL system.²¹

Acknowledgements

We thank the Junta de Andalucía for financial support (Acciones Coordinadas).

References

1. T. G. Spiro and W. M. Stigliani, in Chemistry of the Environment, Prentice Hall, Upper Saddle River, New Jersey, 1996, ch. 2.
2. (a) S. L. Wells and J. DeSimone, Angew. Chem., Int. Ed., 2001, 40, 518; (b) X. Yin and J. R. Moss, Coord. Chem. Rev., 1999, 181, 27; (c) D. H. Gibson, Chem. Rev., 1996, 96, 2063; (d) K. Tanaka, Adv. Inorg. Chem., 1995, 43, 409; (e) P. G. Jessop, T. Ikariya and R. Noyori, Chem. Rev., 1995, 95, 259; (f) W. Leitner, Angew. Chem., Int. Ed. Engl., 1995, 34, 2207.
3. (a) J. C. Calabrese, T. Herskovictz and J. B. Kinney, J. Am. Chem. Soc., 1983, 105, 5914; (b) H. Tanaka, H. Nagano, S. Peng and K. Tanaka, Organometallics, 1992, 11, 1450.
4. For some examples of κ²-C,O–M, see: (a) M. Aresta, C. F. Nobile, V. G. Albano, E. Forni and M. Manassero, J. Chem. Soc., Chem. Commun., 1975, 36; (b) M. Aresta and C. F. Nobile, J. Chem. Soc., Dalton Trans., 1977, 708; (c) G. S. Bristow, P. B. Hitchcock and M. F. Lappert, J. Chem. Soc., Chem. Commun., 1981, 1145; (d) S. Gambarotta, C. Floriani, A. Chiesi-Villa and C. Gaustini, J. Am. Chem. Soc., 1985, 107, 2985.
5. (a) R. Alvarez, E. Carmona, J. M. Marín, M. L. Poveda, E. Gutiérrez-Puebla and A. Monge, J. Am. Chem. Soc., 1986, 108, 2286; (b) E. Carmona, A. K. Hughes, M. A. Muñoz, D. O'Hare, P. J. Pérez and M. L. Poveda, J. Am. Chem. Soc., 1991, 113, 9210.
6. (a) G. Fachinetti, C. Floriani, A. Chiesi-Villa and C. Gaustini, J. Am. Chem. Soc., 1979, 101, 1767; (b) J. C. Bryan, S. J. Geib, A. L. Rheingold and J. M. Mayer, J. Am. Chem. Soc., 1987, 109, 2826.
7. (a) J. Chatt, M. Kubotta, G. J. Leigh, T. C. March, R. Mason and D. J. Yarrow, J. Chem. Soc., Chem. Commun., 1974, 1033; (b) S. Inoue and N. Yamazaki, Organic and Bioinorganic Chemistry of Carbon Dioxide, Halsted Press, Tokyo, Japan, 1982; (c) A. D. Palmer and R. Van Edik, Chem. Rev., 1983, 83, 651; (d) L. K. Fong, J. R. Fox and N. J. Cooper, Organometallics, 1987, 6, 223.
8. (a) R. Alvarez, E. Carmona, A. Galindo, E. Gutiérrez, J. M. Marín, A. Monge, M. L. Poveda, C. Ruíz and J. M. Savariault, Organometallics, 1989, 8, 2430; (b) R. Alvarez, J. L. Atwood, E. Carmona, P. J. Pérez, M. L. Poveda and R. D. Rogers, Inorg. Chem., 1991, 30, 1493.
9. (a) E. Carmona, P. Palma, M. Paneque, M. L. Poveda, E. Gutiérrez-Puebla and A. Monge, J. Am. Chem. Soc., 1986, 108, 6424; (b) E. Carmona, E. Gutiérrez-Puebla, J. M. Marín, A. Monge, M. Paneque, M. L. Poveda and C. Ruíz, J. Am. Chem. Soc., 1989, 111, 2883; (c) E. Carmona, J. M. Marín, P. Palma, M. Paneque and M. L. Poveda, Inorg. Chem., 1989, 28, 1895; (d) M. K. Reinking, J. Ni, P. E. Fanwick and C. P. Kubiak, J. Am. Chem. Soc., 1989, 111, 6454.
10. E. Sánchez-Marcos, R. Caballol, G. Trinquier and J.-C. Barthelat, J. Chem. Soc., Dalton Trans., 1987, 2373.
11. T. Herskovitz and L. J. Guggenberger, J. Am. Chem. Soc., 1976, 98, 1615.
12. A. A. Danopoulos and P. G. Edwards, Polyhedron, 1989, 8, 1767.
13. (a) A. R. Willis, P. G. Edwards, H. Harman and M. B. Hursthouse, Polyhedron, 1989, 9, 1457; (b) A.-R. H. Al-Soudani, A. S. Batsanov, P. G. Edwards and J. A. K. Howard, J. Chem. Soc., Dalton Trans., 1994, 987; (c) P. G. Edwards, P. W. Read, M. B. Hursthouse and K. M. Abdul Malik, J. Chem. Soc., Dalton Trans., 1994, 971; (d) M. D. Fryzuk, T. S. Haddad and S. J. Rettig, Organometallics, 1991, 10, 2026; (e) M. D. Fryzuk and C. D. Montmongery, Coord. Chem. Rev., 1989, 95, 1; (f) C. Bianchini, D. Masi, A. Romerosa, F. Zanobini and M. Peruzzini, Organometallics, 1999, 18, 2376; (g) C. Bianchini, M. Peruzzini, F. Zanobini, C. Lopez, I. de los Ríos and A. Romerosa, Chem. Commun., 1999, 443; (h) F. G. N. Cloke, P. B. Hitchcock and J. B. Love, J. Chem. Soc., Dalton Trans., 1995, 25; (i) R. R. Schrock, A. L. Casado, J. T. Goodman, L.-Ch. Liang, P. J. Bonitatebus, Jr. and W. M. Davis, Organometallics, 2000, 19, 5325.
14. F. A. Cotton, D. J. Darensbourg, S. Klein and B. W. S. Kolthammer, Inorg. Chem., 1982, 21, 2661.
15. L. Pauling, The Nature of the Chemical Bond, Cornell University Press, New York, 3rd edn., 1960.
16. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, John Wiley and Sons, New York, 5th edn., 1997.
17. R. Mathieu, M. Lenzi and R. Poilblanc, Inorg. Chem., 1970, 9, 2030.
18. B. M. Gatehouse and P. Leverett, J. Chem. Soc. A, 1969, 848.
19. I. Castro-Rodríguez, H. Nakai, L. N. Zakharov, A. L. Rheingold and K. Meyer, Science, 2004, 305, 1757.
20. International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, UK, 1974, vol. IV, p. p. 72.
21. Software for the SMART System, V5.04, and SHELXTL, V5.1, Bruker–Siemens Analytica X-ray Instrument, Inc., Madison, WI, 1998.

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Footnote

† CCDC reference numbers 231960 (1) and 231961 (4). See http://www.rsc.org/suppdata/nj/b4/b409385b/ for crystallographic data in .cif or other electronic format.

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