Isomerism in Molybdenum Carbonyl Phosphine Complexes

Isomerism in moment Carbonyl Phosphine ComplexesPart A Preparation and Identification of the Isomers of Mo(CO)4(PPh3)2IntroductionMolybdenum carbonyl phosphine multif propeloriales with the general formula, Mo(CO)4L2 where (L=PR3 R=Me, Ph etc) have an octahedral geometry. This means the complexes atomic number 18 fitted to display either cis or trans stereochemistry. Interconversion between the two isomeric forms is enabled by thermal Mo-P bond cleavage. Only the thermodynamically s dining table isomer of Mo(CO)4(PPh3)2 is formed by direct reaction of Mo(CO)6 and PPh3. So in order to isolate both isomeric forms of the complex, Mo(CO)4(piperidine)2 is used as an intermediate.The aim of this experiment wasThe preparation of Mo(CO)4(piperidine)2 by reaction of Mo(CO)6 with piperidine dissolved in tolueneThe preparation of cis-Mo(CO)4(PPh3)2 by substitution of piperidine with PPh3The thermal isomerization of Mo(CO)4(PPh3)2 to produce the trans isomeric formCharacterization of the p roducts by IR spectroscopy allowed the isomeric forms of severally of the complexes to be identified, and the most stable form of the product to be deduced.Reaction SchemeMethodPreparation of Mo(CO)4(piperidine)2A summary of the preparative details for the formation of Mo(CO)4(piperidine)2 is detailed in table 1.1 multitude saddle hero sandwich MassMolar AmountEquivalenceDensitycm-3gg mol-1molg mL-1Mo(CO)61.00264.003.788E-031.00Toluene15.0012.9892.141.408E-0137.180.865Piperidine10.008.6285.151.012E-0126.730.862Table 1.1 The preparative details of Mo(CO)4(piperidine)2Mo(CO)6 (1.00 g, 3.79 mmol) was dissolved in a admixture of toluene (15 mL) and piperidine (10 mL, 9.62 g, 101.00 mmol), low an unemployed atmosphere with stirring at 110 C for 2 hours under reflux. The resultant white-livered mixture was filtered under vacuum cleaner for 15 minutes, washed with ice-cold 60/80 petroleum ether (210 mL), to yield a yellow crystalline solid of Mo(CO)4(piperidine)2. fork turn up (1.1 9 g, 83%) max/ cm-1 3250.88 (N-H), 2931.85, 2853.10 (C-H), 2011.73, 1877.24, 1756.58, 1706.11 (C=O), 1476.99, 1462.33 (C-C)In order to calculate the yield of Mo(CO)4(piperidine)2 formed, the following method was employed, with details summa getd in table 1.2.Mass ObtainedMolar MassMoles sup loudnessal way outPercentage Yieldgg mol-1molgMo(CO)4(piperidine)21.19378.303.146E-031.43383.05Table 1.2 The yields for the formation of Mo(CO)4(piperidine)2The theoretical mass of Mo(CO)4(piperidine)2 was calculated from its molar mass and the human activity of moles of the limiting reagent, Mo(CO)6 , using the following equation eq.1.1On formation of Mo(CO)4(piperidine)2 the yield of the complex was actually obtained to be 1.19 g. So in order to work out the percentage yield of Mo(CO)4(piperidine)2 formed, eq 1.2 was usedPreparation of Mo(CO)4(PPh3)2 (Isomer A) by Substitution of Piperidine with PPh3A summary of the details of preparing Mo(CO)4(PPh3)2 (isomer A) is given in table 1.3VolumeWe ightMolar MassMolar AmountEquivalenceDensity/ g mL-1cm-3gg mol-1molg mL-1Mo(CO)4(piperidine)20.50378.301.322E-031.00PPh30.75262.292.859E-032.16CH2Cl210.0013.2584.931.560E-01118.041.325Mo(CO)4(piperidine)2 (0.50 g, 1.32 mmol), triphenylphosphine (0.75 g, 2.86 mmol) and CH2Cl2 (10 mL) were refluxed at 40 C, under an inert atmosphere for 15 minutes. The reaction mixture was allowed to cool to room temperature. Then methanol (15 mL) was added to the mixture and cooled in the freezer for 30 minutes. The precipitate was filtered under vacuum for 15 minutes which yielded a pale-yellow solid of Mo(CO)4(PPh3)2(isomer A). Yield (0.43 g, 44%) max/ cm-1 2925.53 (C-H), 2011.26, 1876.74, 1756.04, 1706.81 (C=O), 1462.87 (C=C).The yields of the product, Mo(CO)4(PPh3)2 (isomer A) are summarized in table 1.4.Mass ObtainedMolar MassMolesTheoretical YieldPercentage Yieldgg mol-1molgMo(CO)4(PPh3)2(Isomer A)0.43732.585.870E-040.96844.41As onwards using eq.1.1 and eq.1.2 the percentage yield was calculat ed from the theoretical yield. withal in this case the limiting reagent was found to be Mo(CO)4(piperidine)2Preparation of Mo(CO)4(PPh3)2 (Isomer B) by Thermal Isomerisation of Isomer ATable 1.5 shows a summary of the preparative details of Mo(CO)4(PPh3)2 (isomer B)VolumeWeightMolar MassMolar AmountEquivalenceDensitycm-3gg mol-1molg mL-1Mo(CO)4(PPh3)2 (Isomer A)0.40732.585.460E-041.00Toluene4.003.4692.143.755E-0268.770.865Since isomer A was produced in a relatively low yield, the thermal isomerisation reaction was scaled so that plainly 0.4 g of isomer A was used.Mo(CO)4(PPh3)2 (isomer A) (0.40 g, 0.55 mmol) was added to toluene (4 mL) and stirred under reflux at 110 C for 30 minutes under an inert atmosphere. The solution was cooled to room temperature and 60/80 petroleum ether (9 mL) was added to aid precipitation. The resultant mixure was filtered under vacuum for 15 minutes, rinsed with 60/80 petroleum ether (2 x 5 mL), which yielded a brown solid of Mo(CO)4(PPh3)2 (isomer B). Yield (0.26 g, 65%) max/ cm-1 3056.60 (C-H), 1873.38 (C=O), 1476.92, 1431.71 (C=C).The yields of the product, Mo(CO)4(PPh3)2 (isomer B) are summarized in table 1.6Mass ObtainedMolar MassMolesTheoretical YieldPercentage Yieldgg mol-1molgMo(CO)4(PPh3)2(Isomer B)0.26732.583.549E-040.40065.00As before using eq.1.1 and eq.1.2 the percentage yield was calculated from the theoretical yield.Results and DiscussionThis experiment involved the isolation of the two geometric isomers of Mo(CO)4(PPh3)2. Only one of these isomers can be isolated from the direct reaction of PPh3 with Mo(CO)6. This reaction requires a long reaction time and high temperatures but yields the more thermodynamically stable geometrical isomer. So in order to isolate the isomer B, an alternative synthetic route was employed. A precursor in the form of Mo(CO)4(piperidine)2 was used to yield isomer B, since PPh3 is now able to quickly substitute piperidine. This method relies on the nature of the ligands. Piperidine is a weak field ligand, and so forms a relatively weak dative bond with the molybdenum ion. However, PPh3 is a strong field ligand and so binds strongly with the central metallic element ion. This means PPh3 easily displaces the piperidine ligands to give rise to the isomer B form of the complex Mo(CO)4(PPh3)2.The first leave of the reaction involved the preparation of the intermediate Mo(CO)4(piperidine)2 which is a yellow solid which is in agreement with literature. The complex was formed in good yield at 83%. The invisible spectrum of Mo(CO)4(piperidine)2 shows four peaks corresponding to C=O vibrations at 2011.73, 1877.24, 1756.58 and 1706.11 cm-1 (lit). This suggests the isomer of the complex formed is the cis geometrical isomer. This can be explained by employing a pigeonholing theory technique, which uses an unshifted C=O bonds procedure, as illustrated in figure.EC2(xz)(yz)CO4022Cis-Mo(CO)4(piperidine)2 has C2V symmetry and the irreducible representation of unshifted bonds is deduced to be 2A1 + B1 + B2. Since all of these operations are IR active, four peaks are expected in the carbonyl region of the spectrum.In the IR spectrum there is also a peak at 3250.88 which corresponds to an N-H bond. This is indicative of unreacted piperidine. The presence of residual piperidine did not negatively affect the experiment to any significant extent.The synthesis of Mo(CO)4(PPh3)2 (isomer A) yielded a pale yellow solid as reported by. The complex produced a 44% yield which affected the proceedings of the reaction as enough product was not formed. This meant the ulterior reaction had to be scaled down. The IR spectrum of Mo(CO)4(PPh3)2 (isomer A) displayed four peaks in the carbonyl region at 2011.26, 1876.74, 1756.04 and 1706.81 cm-1 which were similar to those reported in the literature. The compound was also impelled to have C2V symmetry which would therefore be expected to give rise to 4 peaks in the spectrum. Therefore isomer A is cis-Mo(CO)4(PPh3)2.As a resu lt, it can be deduced that isomer B of Mo(CO)4(PPh3)2 is the trans geometrical isomer. The trans isomer has a D4h confidential information concourse and so is expected to produce a single peak in the IR spectrum. As predicted, the spectrum of isomer B has a peak in the carbonyl region at only 1873.38 cm-1. The trans-Mo(CO)4(PPh3)2 isomer was a brown solid and produced at a reasonable yield of 65%.The trans isomeric form of Mo(CO)4(PPh3)2 is the thermodynamically more stable form of the two isomers. The trans isomer required more vigorous reaction condition such as the firmness of purpose heated to 110 C and a longer reaction time as opposed to the milder conditions used for the formation of the cis isomer. The trans isomer places the two bulky PPh3 ligands as farther as possible from one another, at 180 apart, which gives the complex the lowest possible energy.Cis-Mo(CO)4(PPh3)2 is the kinetic product of the reaction since it forms at a faster rate. This is because the energizi ng energy barrier is much smaller, as reflected by the mild conditions imposed (40 C toluene, 15 minute reflux).ConclusionThe synthesis of all three complexes was successful. A good yield was obtained for the formation of Mo(CO)4(piperidine)2 at 83%. However, isomer A of Mo(CO)4(PPh3)2 was produced at a relatively low yield of 44%, in comparison to 65% produced for isomer B. The low yield of isomer A affected the proceedings of the subsequent steps of the reaction, and so required a scaling down of reagents used from this stay onwards.The infrared spectrum of Mo(CO)4(piperidine)2 produced four C=O stretching bands at 2011.73, 1877.24, 1756.58 and 1706.11 cm-1 which matched those reported in literature. This suggested that the complex had a cis geometry and therefore the point group of the complex is C2V.For isomer A of Mo(CO)4(PPh3)2, the point group was also inferred to be C2V. using the method of unshifted C=O bonds, it was determined that four peaks are expected in the spectrum corresponding to C=O vibrations. The infrared spectrum obtained did indeed have four bands at 2011.26, 1876.74, 1756.04 and 1706.81 cm-1. This shows good correlation with the predictions from group theory.For isomer B of Mo(CO)4(PPh3)2, the spectrum displayed a single peak at 1873.38 cm-1 which corresponds to C=O. The geometry of the complex is therefore confirmed to be trans, and the point group deduced to be D4h for the complex.The colours of the complexes showed good agreement with that reported in literature. The intermediate complex Mo(CO)4(piperidine)2 was yellow in colour. The cis isomer of Mo(CO)4(PPh3)2 was pale yellow, whilst the trans isomer was brown.To summarize the products were produced in reasonable yield, and the IR spectra along with group theory allowed the distinction of the isomeric forms of the complexes.QuestionsA complex ion is formed when a d-block revolution metal ion forms a coordinate bond by accepting a pair of electrons from a ligand. A ligand is an io n or molecule that surrounds the metal ion and is able to give a pair of electrons to it. This means the ligand is a Lewis base, whilst the central metal ion functions as a Lewis acid.There are two types of bonding between a d-block spiritual rebirth metal and a ligand, namely -bonding and -bonding. -bonding is present in all interactions between a metal and ligand because the solitary pair of the ligand lies on the internuclear axis. However, some ligands are able to participate in -bonding interactions. A ligand is said to be a -donor ligand if the interaction involves donation of electron density from a filled orbital towards an renounce metal orbital. It is also possible for a ligand to be a -acceptor ligand. In this case, electron density is donated from a filled orbital on the metal to an empty orbital on the ligand.The bonding between a d-block transition metal and specific ligands is discussed belowCOCarbon monoxide is a strong field ligand with its position high in the spectrochemical series, which means that it gives rise to low spin complexes, due to the fact that it has a large value of oct. There are two components that hear the metal carbonyl bonding.-bonding when a lone pair of electrons from CO is donated to an empty d-orbital on the metal, this is known as -bonding.-acceptor ligand when electrons from the filled metal d-orbital are donated to an empty * acceptor orbital on CO, this is known as -back donation.CO is both a -donor and -acceptor ligand. Since these components complement one another, this results in synergic bonding. In other words, the greater the -donation the greater the -back donation. However the extent of backbonding does depend on the oxidation state of the metal and the electronic properties of the other ligands present. Structurally, as a result of synergic reinforcing components, the metal carbonyl bond strength is increased, but this means the CO is weakened, relative to free CO gas. This is due to an increase in the electron density of the antibonding * orbital.PPh3Triphenylphosphine has a lone pair of electrons on P that it is able to donate to the transition metal centre, and so acts as a -donor ligand as illustrated in figure.The phosphine has a slothful orbital, so in theory it can also act as a -acceptor ligand. However, the 3d orbitals that are vacant on phosphorus may be of too high energy for -backdonation to occur. Instead the * orbital contributes as the main acceptor component.The ability of a phosphine ligand to act as an acceptor is controlled by the identity of the R substituent. Since in PPh3, the substituent is phenyl which is not a very electron-withdrawing group, the acceptor properties are weak. This is because the phenyl rings are not electron-withdrawing enough to lower the energy of the 3d orbitals on phosphorus. However, if the R substituent was replaced by an electron-withdrawing group, the acceptor properties of the phosphine group would be greater. To summarise, PPh3 is a good donor but a poor acceptor.PiperidineThe molecular formula of piperidine is C5H10NH and the structural formula is displayed in figure.Piperidine can act as a ligand because the nitrogen atom has a lone pair of electrons. However unlike CO and PPh3 it is classed as an intermediate field ligand. Piperidine is able to donate its lone pair of electrons on nitrogen to the transition metal centre, and so is said to act as a -donor. However since the nitrogen atom only has one lone pair of electrons, it is unable to participate in -backdonation.Mo(CO)4L2 where L exists as piperidine or PPh3 may exist as two different geometrical isomers cis or trans. By considering only the CO bond of the compound in the infrared spectrum and by exploitation of group theory, the molecular arrangement of the compounds fain could be determined.CisFor cis-Mo(CO)4L2 the point group was identified to be C2V (since it has a C2 axis, (xz) and (yz) components).In the case of cis-Mo(CO)4L2 the chara cter table for C2V isCHARACTER TABLE C2V hereSince only the CO bonds are existence considered, the reducible representation of the number of unshifted CO bonds under each operation of the point group is detailed in table.EC2(xz)(yz)CO4022The reduction formula provides a means of converting a reducible representation into a sum of irreducible representationsWhere ai= number of times an IRREP contributes to the reducible representation h= total number of symmetry operations, nR= number of operations in a class (R) = character in reducible representation IR= character in IRREPTherefore when the formula is applied to each line of the character table we getSo overall CO = 2A1+B1+B2The complex was then described in terms of vibrational modes, COEC2(xz)(yz)CO2011Using table (C2v character table) it can be seen that A1, B1 and B2 are all infrared active. Therefore cis-Mo(CO)4L2 would be expected to give rise to four peaks in the vibrational spectrum.The same process was carried out for the trans isomer in order to determine the number of IR active bands in the vibrational spectrum.TransFor trans-Mo(CO)4L2 the point group was identified to be D4hIn the case of trans-Mo(CO)4L2 the character table for D4h isCHARACTER TABLE D4h hereSince only the CO bonds are being considered, the reducible representation of the number of unshifted CO bonds under each operation of the point group is detailed in table.E2C4C22C22C2i2S4h2i2d3N4002000420As before the reduction formula was used to convert the reducible representation into a sum of irreducible representationsTherefore when the formula is applied to each line of the character table we getE2C4C22C22C2i2S4h2i2daiA1g40040004401A2g400-4000-4