Kant did indeed write the words you quote. But I wonder whether you've actually read them in their context, because it seems very obvious to me that he is applying this statement only to metaphysics. He is saying that we cannot know for certain (for instance) whether God exists, or whether "free will" is real. Now, this may be right or wrong, but it certainly isn't anti-rational.
As for the "wedge between the natural world and the human mind": if by that you mean that, e.g., he insisted that our minds aren't presented directly with Things Themselves, and that perception is an indirect affair: yup, he did, and he was right, and again I see nothing anti-rational in that. (Yes, I know Objectivism teaches otherwise. So much the worse for Objectivism.)
As for the "wedge between the natural world and the human mind": if by that you mean that, e.g., he insisted that our minds aren't presented directly with Things Themselves, and that perception is an indirect affair: yup, he did, and he was right, and again I see nothing anti-rational in that. (Yes, I know Objectivism teaches otherwise. So much the worse for Objectivism.)