The American Colonization Society was formed in 1820 with the express purpose of carving out a place in Africa for freed slaves from the United States. Progress was slow, but in 1847 the independent Republic of Liberia was formed. There were thirty small indigenous languages spoken by the natives of Liberia, but the Capital city of Liberia and the political structure of Liberia was dominated by American-Liberians, who were former slaves that had gained their freedom. When the US Navy captured slave ships, trying to smuggle slaves into the US, these slaves were taken to Liberia. After 1847, the United States pretty much lost interest in Liberia. It was never an official American colony. There were few slaves in the United States that wanted to go back to Africa and nobody was willing to pay to transport the bulk of the slaves in the USA back to Liberia anyway.
The Civil war of 1861 to 1865 was a costly and bloody war. It was followed by Reconstruction, which dominated the attention of the Federal government, until 1876. This was all before the Scramble for Africa began in earnest. The United States did send a low level representative to the Berlin Conference of 1885, that laid the ground rules for how the Europeans would carve up Africa. The Americans played a limited role in that conference. Their suggestions that Africa remain open for trade and that some major rivers in Africa become internationalized, fell on deaf ears.
After 1876, the United States paid more attention to settling the vast territory west of the Mississippi river. It did not need African colonies, since it had so much open land that was already under its political control, but sparsely populated. By the time the western frontier was closed and the last three states (Arizona, New Mexico and Oklahoma) were admitted to the United States, the Scramble for Africa was over.
Sources: "The Scramble for Africa" by Thomas Pakenham
"The Growth of the American Republic: Volume 2, 1805 to 1950" by Samuel Elliot Morrison and Henry Commager

I am not an expert on the period, but a point that should be cited is probably the existence of the Monroe Doctrine.

It was a very important part of the foreign policy of the United States from 1823, when it was stated by president James Monroe during the state of the union declaration.

The main point was that intervention in the Americas by European powers would be seen by the US as hostile. Keep in mind that the doctrine was announce shortly after the majority of latin american countries had proclaimed independence from Spain.

From the state of the Union address: "The occasion has been judged proper for asserting, as a principle in which the rights and interests of the United States are involved, that the American continents, by the free and independent condition which they have assumed and maintain, are henceforth not to be considered as subjects for future colonization by any European powers."

A corollary of the Monroe doctrine was that, while the US claimed that the European powers should not meddle in the New World, the US would not meddle in the Old World.